Add Protocol Gateway Template and Example

Add a template for MQTT protocol gateways and an example implementation
of this template that supports the MQTT API of the Azure IoT Hub.

Signed-off-by: Abel Buechner-Mihaljevic <[email protected]>

.gitignore

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+**/target/
+**/.idea/
+**/*.iml
+**/.vscode/
+**/bin/
+**/.vertx/
+**/.factorypath
+
+# Eclipse files
+**/.classpath
+**/.project
+**/.settings/
+.checkstyle
+
+# OS files
+.DS_Store

protocol-gateway/README.md

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+# Hono Protocol Gateways
+
+Contains [protocol gateways](https://www.eclipse.org/hono/docs/concepts/connecting-devices/#connecting-via-a-protocol-gateway) for Eclipse Hono™.
+
+The directory [mqtt-protocol-gateway-template](mqtt-protocol-gateway-template) provides a template for the creation of 
+custom MQTT protocol gateways.
+
+The directory [azure-mqtt-protocol-gateway](azure-mqtt-protocol-gateway) contains an example implementation of the
+MQTT protocol gateway template that provides (parts of) the MQTT interface of Azure IoT Hub. 

protocol-gateway/azure-mqtt-protocol-gateway/README.md

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+# "Azure IoT Hub" Protocol Gateway
+
+This Protocol Gateway shows how to use Hono's Protocol Gateway Template to implement a production-ready protocol gateway. 
+The MQTT-API of "Azure IoT Hub" serves as a working example. Parts of its API are mapped to Hono's communication patterns.
+
+Full compatibility with the Azure IoT Hub is not a design goal of this example. It is supposed to behave similarly for 
+the "happy path", but cannot treat all errors or misuse in the same way as the former.
+
+Supported are the following types of messages:
+
+## Mapping of Azure IoT Hub messages to Hono messages
+
+**Device-to-cloud communication**
+
+| Azure IoT Hub message | Hono message | Limitations |
+|---|---|---|
+| [Device-to-cloud](https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-devguide-d2c-guidance) with QoS 0 (*AT MOST ONCE*) | [Telemetry](https://www.eclipse.org/hono/docs/api/telemetry/#forward-telemetry-data) | Messages are not brokered | 
+| [Device-to-cloud](https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-devguide-d2c-guidance) with QoS 1 (*AT LEAST ONCE*) | [Event](https://www.eclipse.org/hono/docs/api/event/#forward-event) | Messages are not brokered | 
+
+
+**Cloud-to-device communication**
+
+| Hono message | Azure IoT Hub message | Limitations |
+|---|---|---|
+| [One-way Command](https://www.eclipse.org/hono/docs/api/command-and-control/#send-a-one-way-command) | [Cloud-to-device](https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-devguide-messages-c2d) | Messages are not brokered (ignores CleanSession flag) | 
+| [Request/Response Command](https://www.eclipse.org/hono/docs/api/command-and-control/#send-a-request-response-command) | [Direct method](https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-devguide-direct-methods) | |
+
+## Limitations
+
+Not supported are the following features of Azure IoT Hub:
+
+ * "device twins"
+ * file uploads 
+ * message brokering 
+ * "jobs"
+ * the back-end application API 
+ * device authentication with client certificates
+
+## Device Authentication
+
+A Hono protocol gateway is responsible for the authentication of the devices.
+This example implementation does not provide or require data storage for device credentials. 
+Instead, it can only be configured to use a single demo device, which must already be present in Hono's device registry (see below).
+Client certificate based authentication is not implemented.
+
+Since there is only one device in this example implementation anyway, the credentials for the tenant's gateway client are not looked up dynamically, but are taken from the configuration.
+
+
+## Prerequisites
+
+### Registering Devices
+
+The demo device and the gateway need to be registered in Hono's device registry. For the gateway, credentials must be created. 
+The [Getting started](https://www.eclipse.org/hono/getting-started/#registering-devices) guide shows how to do this.
+
+Alternatively, the script `scripts/create_demo_devices.sh` can be used to register the devices and create credentials:
+~~~sh
+# in directory: protocol-gateway/azure-mqtt-protocol-gateway/scripts/
+bash create_demo_devices.sh
+~~~
+
+After completion the script prints the configuration properties. Copy the output into the 
+file `protocol-gateway/azure-mqtt-protocol-gateway/src/main/resources/application.properties`. 
+
+
+### Configuration
+
+The protocol gateway needs the configuration of:
+
+1. the AMQP adapter of a running Hono instance to connect to
+2. the MQTT server 
+3. the demo device to use.
+
+By default, the gateway will connect to the AMQP adapter of the [Hono Sandbox](https://www.eclipse.org/hono/sandbox/).
+However, it can also be configured to connect to a local instance.
+The default configuration can be found in the file `protocol-gateway/azure-mqtt-protocol-gateway/src/main/resources/application.properties` 
+and can be customized using [Spring Boot Configuration](https://docs.spring.io/spring-boot/docs/current/reference/html/spring-boot-features.html#boot-features-external-config). 
+
+
+### Starting a Receiver
+
+Telemetry and event messages need an application that consumes the messages. 
+The [Getting started](https://www.eclipse.org/hono/getting-started/#starting-the-example-application) guide shows how to start the example application that receives the messages.
+
+
+## Starting the Protocol Gateway
+
+Build the template project:
+~~~sh
+# in directory: protocol-gateway/mqtt-protocol-gateway-template/
+mvn clean install
+~~~
+
+and start the protocol gateway:
+~~~sh
+# in directory: protocol-gateway/azure-mqtt-protocol-gateway/
+mvn spring-boot:run
+~~~
+
+
+## Enable TLS 
+
+Azure IoT Hub only provides connections with TLS and only offers port 8883. To start the protocol gateway listening
+on this port with TLS enabled and a demo certificate, run:
+
+~~~sh
+# in directory: protocol-gateway/azure-mqtt-protocol-gateway/
+mvn spring-boot:run -Dspring-boot.run.profiles=ssl
+~~~
+**NB** Do not forget to build the template project before, as shown above.
+
+With the [Eclipse Mosquitto](https://mosquitto.org/) command line client, for example, sending an event message would then look like this:
+
+~~~sh
+# in directory: protocol-gateway/azure-mqtt-protocol-gateway
+mosquitto_pub -d -h localhost -p 8883 -i '4712' -u 'demo1' -P 'demo-secret' -t "devices/4712/messages/events/" -m "hello world" -V mqttv311 --cafile target/config/hono-demo-certs-jar/trusted-certs.pem
+~~~
+
+Existing hardware devices might need to be configured to accept the used certificate. 
+
+## Example Requests
+
+With the [Eclipse Mosquitto](https://mosquitto.org/) command line client the requests look like the following.
+
+**Telemetry**
+
+~~~sh
+mosquitto_pub -d -h localhost -i '4712' -u 'demo1' -P 'demo-secret' -t 'devices/4712/messages/events/?foo%20bar=b%5Fa%5Fz' -m "hello world" -V mqttv311 -q 0
+~~~
+
+**Events**
+
+~~~sh
+mosquitto_pub -d -h localhost -i '4712' -u 'demo1' -P 'demo-secret' -t 'devices/4712/messages/events/?foo%20bar=b%5Fa%5Fz' -m '{"alarm": 1}' -V mqttv311 -q 1
+~~~
+
+### Commands 
+
+The example application can be used to send commands. 
+The [Getting started](https://www.eclipse.org/hono/getting-started/#advanced-sending-commands-to-a-device) shows a walk-through example.
+
+**Subscribe for one-way commands**
+
+~~~sh
+mosquitto_sub -v -h localhost -u "demo1" -P "demo-secret" -t 'devices/4712/messages/devicebound/#' -q 1
+~~~
+
+**Subscribe for request-response commands**
+
+~~~sh
+mosquitto_sub -v -h localhost -u "demo1" -P "demo-secret" -t '$iothub/methods/POST/#' -q 1
+~~~
+
+When Mosquitto receives the command, the output in the terminal should look like this: 
+~~~sh
+$iothub/methods/POST/setBrightness/?$rid=0100bba05d61-7027-4131-9a9d-30238b9ec9bb {"brightness": 87}
+~~~
+
+**Respond to a command**
+
+When sending a response, the request id must be added. The ID after `rid=` can be copied from the received message 
+and pasted into a new terminal to publish the response like this:
+~~~sh
+export RID=0100bba05d61-7027-4131-9a9d-30238b9ec9bb
+mosquitto_pub -d -h localhost -u 'demo1' -P 'demo-secret' -t "\$iothub/methods/res/200/?\$rid=$RID" -m '{"success": true}' -q 1
+~~~
+Note that the actual identifier from the command must be used.