Allow to find a suitable volume control when using role based policy linking

When using role based priorities a volume slider presented to the user should adjust the volume of the currently playing role.

E.g. when a phone has an incoming call and is ringing the default volume slider should adjust the ringing volume and once the call has been picked up it should adjust the call volume and once the call ended it should adjust the media volume again.

It's currently hard for e.g. desktop shells to find out what a suitable sink for volume control is so add an example script that can be used with media-role-nodes.conf to find a suitable target for volume control by storing it's name in the metadata.

The background for this is that I'd like to have phosh's volume slider (and the phone's volume buttons) pick up the right volume depending on context (While the settings app has separate volume sliders per role).

I hope this sounds like a somewhat sane approach? I'm pretty sure this can be improved considerably so I'd be happy about any suggestions.