Add mqtt-client components

[ERnsTL]

• Maybe also useful for integration with MsgFlo
  • TODO separate step - may need an additional component "msgflo-adapter"
• TCP-based, but can also run over Unix domain sockets or WebSockets
• Requires TLS for security
• transport and security should be configurable via one string -> connection URL
• Publish-Subscribe pattern, message broker model
• on top of that Request-Response is possible, but that is not always the use-case and then graph hacks would be necessary just to avoid "port unconnected" error
• Decision: make it 2 separate components (publisher and subscriber)

Client:

• crate: https://github.com/bytebeamio/rumqtt

Server for testing:

• https://github.com/fhmq/hmq
• https://github.com/bytebeamio/rumqtt (has client+server crate)
• RabbitMQ
• own Mosquitto instance
• public test instance: https://test.mosquitto.org port 1883 or better 8886 (unauthenticated, TLS, LE certificate)

Implementation:

• DONE add ability to select the topic to publish/subscribe to - not possible via URL it seems, but we could pack it in.

[ERnsTL]

Pulling this forward in planning to testdrive new arrayports features