support for "foreign" snapshots and/or custom naming schemes

As I understand it, currently btrbk is designed to only work on snapshots it has created itself, or at least ones that follow the configured -- or is it any supported? -- naming scheme. This leads to duplicated functionality (= added complexity = more ways to fail) when snapshots created by other tools and/or manually created ones are added to the mix.

For example, I already use [Timeshift](https://github.com/teejee2008/timeshift) for system-level undo on Ubuntu and Linux Mint -- it's well integrated and works well. That means I already have scheduled (rw) snapshots of the root subvolume, I just need btrbk on the backup server to [re-shapshot them ro] to pull them. Then I have a couple of subvolumes where before-change/after-change makes more sense than a time-based schedule, so I ro-snapshot them manually or via custom scripts, those just need intelligent replication. For the home subvolume, I'd appreciate the full functionality of btrbk, scheduled snapshots, backups, pruning, the works.

Start by reviewing how btrbk identifies and names snapshots, then compare the requested behavior for Timeshift-created and manually created snapshots on root and other subvolumes. Clarify whether foreign snapshots should be re-snapshotted read-only, replicated, scheduled, and pruned, and define the naming and ownership rules before implementation; the issue names no files or tests.

Written by the indexing model from the issue text.