Add Service For Querying If User Has An Active Channel

[gabheadz]

Is your feature request related to a problem? Please describe.
No

Describe the solution you'd like
Add service to the existing API, in order to query if a given user_ref has an active channel.

Describe alternatives you've considered
No alternatives considered so far.

Additional context
Actually there is no way to know if a user_ref has an active channel, and some backend services could want to know beforehand if an user has an active channel, prior to requesting a message delivery.