Add proxy server examples

Add examples of specifying a proxy server for:

- MessageCard format
- Adpative Card format

Thanks to @ahfa92 for the idea and testing feedback.

refs GH-240

README.md

@@ -26,6 +26,7 @@ A package to send messages to a Microsoft Teams channel.
     - [How to create a webhook URL (Connector)](#how-to-create-a-webhook-url-connector)
   - [Examples](#examples)
     - [Basic](#basic)
+    - [Specify proxy server](#specify-proxy-server)
     - [User Mention](#user-mention)
     - [Tables](#tables)
     - [Set custom user agent](#set-custom-user-agent)
@@ -192,6 +193,16 @@ This is an example of a simple client application which uses this library.
 - `MessageCard`
   - File: [basic](./examples/messagecard/basic/main.go)
 
+#### Specify proxy server
+
+This is an example of a simple client application which uses this library to
+route a generated message through a specified proxy server.
+
+- `Adaptive Card`
+  - File: [basic](./examples/adaptivecard/proxy/main.go)
+- `MessageCard`
+  - File: [basic](./examples/messagecard/proxy/main.go)
+
 #### User Mention
 
 These examples illustrates the use of one or more user mentions. This feature

examples/adaptivecard/proxy/main.go

@@ -0,0 +1,89 @@
+// Copyright 2022 Adam Chalkley
+//
+// https://github.com/atc0005/go-teams-notify
+//
+// Licensed under the MIT License. See LICENSE file in the project root for
+// full license information.
+
+/*
+
+This is an example of a client application which uses this library to:
+
+	- generate a basic Microsoft Teams message in Adaptive Card format
+	- submit the message using an explicit proxy server URL
+
+Of note:
+
+- message is in Adaptive Card format
+- default timeout
+- package-level logging is disabled by default
+- validation of known webhook URL prefixes is *enabled*
+- simple message submitted to Microsoft Teams consisting of title and
+  formatted message body
+
+See https://docs.microsoft.com/en-us/adaptive-cards/authoring-cards/text-features
+for the list of supported Adaptive Card text formatting options.
+
+*/
+
+package main
+
+import (
+	"log"
+	"net/http"
+	"net/url"
+	"os"
+
+	goteamsnotify "github.com/atc0005/go-teams-notify/v2"
+	"github.com/atc0005/go-teams-notify/v2/adaptivecard"
+)
+
+func main() {
+
+	// Initialize a new Microsoft Teams client.
+	mstClient := goteamsnotify.NewTeamsClient()
+
+	proxyURLString := "http://proxyIp:proxyPort"
+	proxyUrl, err := url.Parse(proxyURLString)
+
+	if err != nil {
+		log.Printf(
+			"failed to parse proxy URL %q: %v",
+			proxyURLString,
+			err,
+		)
+		os.Exit(1)
+	}
+
+	httpClient := mstClient.HTTPClient()
+	httpClient.Transport = &http.Transport{Proxy: http.ProxyURL(proxyUrl)}
+
+	// Set webhook url.
+	webhookUrl := "https://outlook.office.com/webhook/YOUR_WEBHOOK_URL_OF_TEAMS_CHANNEL"
+
+	// The title for message (first TextBlock element).
+	msgTitle := "Hello world"
+
+	// Formatted message body.
+	msgText := "Here are some examples of formatted stuff like " +
+		"\n * this list itself  \n * **bold** \n * *italic* \n * ***bolditalic***"
+
+	// Create message using provided formatted title and text.
+	msg, err := adaptivecard.NewSimpleMessage(msgText, msgTitle, true)
+	if err != nil {
+		log.Printf(
+			"failed to create message: %v",
+			err,
+		)
+		os.Exit(1)
+	}
+
+	// Send the message with default timeout/retry settings.
+	if err := mstClient.Send(webhookUrl, msg); err != nil {
+		log.Printf(
+			"failed to send message: %v",
+			err,
+		)
+		os.Exit(1)
+	}
+}

examples/messagecard/proxy/main.go

@@ -0,0 +1,73 @@
+// Copyright 2021 Adam Chalkley
+//
+// https://github.com/atc0005/go-teams-notify
+//
+// Licensed under the MIT License. See LICENSE file in the project root for
+// full license information.
+
+/*
+
+This is an example of a client application which uses this library to:
+
+	- generate a Microsoft Teams message in MessageCard format
+	- submit the message using an explicit proxy server URL
+
+Of note:
+
+- message is in MessageCard format
+- default timeout
+- package-level logging is disabled by default
+- validation of known webhook URL prefixes is *enabled*
+- simple message submitted to Microsoft Teams consisting of title and
+  formatted message body
+
+*/
+
+package main
+
+import (
+	"log"
+	"net/http"
+	"net/url"
+	"os"
+
+	goteamsnotify "github.com/atc0005/go-teams-notify/v2"
+	"github.com/atc0005/go-teams-notify/v2/messagecard"
+)
+
+func main() {
+
+	// Initialize a new Microsoft Teams client.
+	mstClient := goteamsnotify.NewTeamsClient()
+
+	proxyURLString := "http://proxyIp:proxyPort"
+	proxyUrl, err := url.Parse(proxyURLString)
+
+	if err != nil {
+		log.Printf(
+			"failed to parse proxy URL %q: %v",
+			proxyURLString,
+			err,
+		)
+		os.Exit(1)
+	}
+
+	httpClient := mstClient.HTTPClient()
+	httpClient.Transport = &http.Transport{Proxy: http.ProxyURL(proxyUrl)}
+
+	// Set webhook url.
+	webhookUrl := "https://outlook.office.com/webhook/YOUR_WEBHOOK_URL_OF_TEAMS_CHANNEL"
+
+	// Setup message card.
+	msgCard := messagecard.NewMessageCard()
+	msgCard.Title = "Hello world"
+	msgCard.Text = "Here are some examples of formatted stuff like " +
+		"<br> * this list itself  <br> * **bold** <br> * *italic* <br> * ***bolditalic***"
+	msgCard.ThemeColor = "#DF813D"
+
+	// Send the message with default timeout/retry settings.
+	if err := mstClient.Send(webhookUrl, msgCard); err != nil {
+		log.Printf("failed to send message: %v", err)
+		os.Exit(1)
+	}
+}