Allow users to disable sync

I'm currently on a plane, and multipass decided to start syncing an image sync in the background, starving all my other processes of network.

Currently, I've just had to run `snap disable multipass`, but I'd prefer to just be able to disable the sync (so that I could continue using multipass with the previously-downloaded images locally, for example).

(Even better would be if I could also defer the sync for a certain amount of time; I know I'll be home in N hours, and that would save me from having to remember to reenable the sync in the future.)

No files, tests, or entry points are named in the issue. Start by tracing how Multipass image synchronization is initiated and exposed to users, then determine where a disable or deferred-sync setting belongs. Done means users can stop background image syncing while continuing to use already-downloaded images, with a documented way to reenable or defer it.

Written by the indexing model from the issue text.