Extend Command and Control API for signalling that no command is currently available

[sysexcontrol]

See the discussion thread "Commands using the HTTP adapter" in the mailing list:

We need to define and implement a mechanism for applications to notify protocol adapters that there is "no command currently available".
This especially targets protocol adapters where devices are usually not permanently connected, but that behave like HTTP with a request-response pattern.

Excerpt from the mailing list discussion: we want to only use this in a protocol adapter, if it was enabled for it at a tenant level. By default, this should just be ignored.

[sophokles73]

@sysexcontrol are you planning to have this implemented in 0.7?

[sysexcontrol]

@sophokles73 : this would be my plan. We can revisit the issue scheduling after the next milestone to see if it matches.