Refresh snaps with needed credentials

In 2.30 we started tracking for each snap the userID of the installing user, snap refresh foo will also set this for a snap that didn’t have yet the information.

The plan now is to use that information to refresh snaps in groups with the credentials of their installing users.

This work addresses: https://bugs.launchpad.net/snapd/+bug/1733910

started proposing:

https://github.com/snapcore/snapd/pull/4392

which also adds a few spread specific about private snaps. There is still some work needed there to turn them on.

code for this has now landed (will be in 2.31), in the end we don’t group but query for all snaps for all users (to make a best effort to respect validation with the current APIs)