Add support for snapd confdb feature - allow setting up instance-wide providers

There are two features here, we can split them later if needed:

1) ability to load system-wide config with provider information
2) ability to consume provider data via snapd confdb interface

Start by reviewing the snap's existing configuration and snapd integration entry points, focusing on how system-wide provider information could be loaded and consumed through the snapd confdb interface. Done means both listed capabilities work for instance-wide providers; the two parts may be split if needed.

Written by the indexing model from the issue text.