Add MQTT Gateway Support

[timvw74]

Starter for MQTT Gateway support.

The Consumer Client keeps a local Systems.Collections.Generic.Queue , due to MQTT's "push" technology.

Not implemented: IAmAMessageConsumer.Acknowledge IAmAMessageConsumer.Reject IAmAMessageConsumer.Requeue

[CLAassistant]

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2 out of 3 committers have signed the CLA.

:white_check_mark: iancooper
:white_check_mark: timvw74
:x: Tim van Wijk

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[iancooper]

Minor remarks though. Once we get this cleaned up, as you are the only person working on it, we can pretty much wave your PRs through if they are just for this transport, with feedback so you can iterate

[iancooper]

@timvw74 I can merge if you would like. We can ignore the failing AWS tests for SNS/SQS

[iancooper]

@timvw74 I converted to a draft whilst we sort out your build

[iancooper]

Just a note that we tend to disable test parallelization for XUnit by putting tests into a collection.