Adds MQTT backend

[alex-oleshkevich]

Based on work done in https://github.com/encode/broadcaster/pull/32/files

Differences:

1. asyncio-mqtt replaced with aiomqtt
2. backend code migrated to support library change (https://sbtinstruments.github.io/aiomqtt/migration-guide-v2.html)

Closes #32

[alex-oleshkevich]

@tomchristie could you please review this?

[lovelydinosaur]

could you please review this?

Not sure - should we be including this in the package, or push it out to a repository / code sample that we can link to? (Perhaps same goes for the Kafka backend too?)

Refs https://github.com/encode/broadcaster/discussions/118

[alex-oleshkevich]

could you please review this?

Not sure - should we be including this in the package, or push it out to a repository / code sample that we can link to? (Perhaps same goes for the Kafka backend too?)

Refs #118

Okay, I will then close this PR w/o merging. As of Kafka - it is already in master.

[lovelydinosaur]

I will then close this PR w/o merging.

Is it worth creating a gist or repo with the MQTT backend, that we link to as an example of a third party implementation?

[alex-oleshkevich]

Not to be merged. Will post it as gist.

[alex-oleshkevich]

Published as gist - https://gist.github.com/alex-oleshkevich/68411a0e7ad24d53afd28c3fa5da468c