Proposal for interface to enable configuration in between snaps

snapd supports snap configuration, which is super handy way to alter snap behaviour from outside, especially when combined with abilities to start stop own services.

However this is at he moment mostly limited to command line interface, other option would be snap control interface, which is for obvious reason bad idea.

This is not ideal when we want to establish light weight communication between two snaps.
Ideally this should be covered by dedicated interface, such an interface should permit snap A change configuration of snap B. Good inspiration could be taken from content interface.