Add configuration for broker to allow clients to publish to '$' topics

[LukasWoodtli]

Changes included in this PR

Add configuration to allow a broker to accept messages published to a topic starting with a dollar sign ($).

Current behavior

Currently, a client is not allowed to publish on topics that start with a dollar sign. This was introduced by #245.

New behavior

Now a configuration is added to allow clients to publish to dollar topics.

Impact

The new configuration is disabled by default. So the current behavior stays as long as the user doesn't change it.

Checklist

1. Does your submission pass the existing tests?
2. Are there new tests that cover these additions/changes?
3. Have you linted your code locally before submission?

[ajmirsky]

@LukasWoodtli Thanks for contributing to the project! Per my read of the mqtt 3.1.1 spec, receiving $ topics is not allowed, configuration or otherwise.

https://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718108

Do you read it differently? I'm hesitant to provide an option that makes the broker/client configurable to be non-compliant. Can you share with me your use case? I'm sure there is a valid use case. I will review and see if I can help find an alternate implementation where we can maintain compliance and still allow your use case.

[ajmirsky]

@LukasWoodtli

would this meet your needs? it would enable your use case without adding something that doesn't comply with the MQTT spec:

https://github.com/ajmirsky/amqtt/blob/feat/allow_dollar_topics/samples/broker_dollar_topics.py