Propagate protocol-specific context to authorization

Propagate protocol-specific context like MQTT client ID to
authentication and authorization backends. The propagation was already
there for topic authorization. Resource check needed propagation and a
new argument in the backend function. Virtual host check just needed
propagation, as there was already a context parameter (for peer
address).

A context parameter and propagation was already there for login-related
functions.

Unfortunately the context API is not fully consistent, due to historical
reasons: proplists are the historical data structure, maps are used
whenever possible.

References #1767

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