Autoconnect requests for pulseaudio

[pedronis]

@jdstrand

What is needed is for snapd to grow a new interface so a snap can plugs it and snapd control can mediate the access to its APIs.

To be clear I think there are two parts to that:

• one is an API or snapctl feature to make the query, something like perhaps:

  snapctl is-connected --pid PID|--security-tag SECTAG slot|plug

  that would let the snap query whether there’s a snap connected to the slot/plug on the other side of that PID/security tag, it might also have to instead tell that PID is unconfined?

• an interface that let a snap access that query capability for its plugs/slots for some set of interfaces, we could also separately decide though that some *-control or *-support interfaces come with the implicit permission for the control slot respectively support plug snap to make queries for the corresponding consumer interface