Support custom SASL mechanisms

[wbarnha]

There is some interest in supporting various SASL mechanisms not
currently included in the library:

• Support for sasl_mechanism="DMS" dpkp/kafka-python#2110 (DMS)
• Native Windows Kerberos authentication via Win32's SSPI API. dpkp/kafka-python#2204 (SSPI)
• Would you consider support for MSK & IAM integration ? dpkp/kafka-python#2232 (AWS_MSK_IAM)

Adding these mechanisms in the core library may be undesirable due to:

• Increased maintenance burden.
• Unavailable testing environments.
• Vendor specificity.

This PR provides a quick prototype for a pluggable SASL system (and potential alternate route to supporting dpkp#2255).

I don't want to be presumptuous about the support for custom SASL extensions on the behalf of the maintainers of kafka-python.
I hope that this quick example can serve as a venue for discussion about the viability of the idea!

---

Example

To define a custom SASL mechanism a module must implement two methods:

def validate_config(conn):
    # Check configuration values, available libraries, etc.
    assert conn.config['vendor_specific_setting'] is not None, (
        'vendor_specific_setting required when sasl_mechanism=MY_SASL'
    )

def try_authenticate(conn, future):
    # Do authentication routine and return resolved Future with failed
    # or succeeded state.

And then the custom mechanism should be registered before initializing
a KafkaAdminClient, KafkaConsumer, or KafkaProducer:

import kafka.sasl
from kafka import KafkaProducer

import my_sasl

kafka.sasl.register_mechanism('MY_SASL', my_sasl)

producer = KafkaProducer(sasl_mechanism='MY_SASL')

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Notes

ABCs

This prototype does not implement an ABC for custom SASL mechanisms.
Using an ABC would reduce a few of the explicit assertions involved with
registering a mechanism and is a viable option. Due to differing feature
sets between py2/py3 this option was not explored, but shouldn't be
difficult.

Private Methods

This prototype relies on some methods that are currently marked as
private in BrokerConnection.

• ._can_send_recv
• ._lock
• ._recv_bytes_blocking
• ._send_bytes_blocking

A pluggable system would require stable interfaces for these actions.

Alternative Approach

If the module-scoped dict modification in register_mechanism feels too
clunky maybe the addtional mechanisms can be specified via an argument
when initializing one of the Kafka* classes?

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This change is 

[wbarnha]

This is so much cleaner. Thank you for writing this.

[wbarnha]

Actually, let's pull in the tests and make life easier to manage this PR.

[wbarnha]

On further though, it would actually be better to pull in changes from #147 into this PR.

[wbarnha]

Resuming in #170.