feat!: Add more locking possibilities

[aawsome]

based on #163

see rustic-rs/rustic#1050

BREAKING CHANGES:

• DeleteOptions in SnapshotFile now has more enum options.
• Repository::save_snapshots now returns the list of Ids of the saved snapshots.
• RepoId now must also impl Debug

Missing points:

• Handle duplicate snapshots correctly - especially create at max. 1 new snapshot per "original" snapshot during lock
• Handle locks in forget command (retention policy: only count one/the original snapshot for retention)
• Handle locks correctly in the tag command (don't even try to remove locked snapshots)

[simonsan]

Blocked on #163: PR requires some changes and maybe a discussion about Lock design as a type state for a backend? Not entirely sure, if that would be a good approach as the expectation on a backend with a lock is not completely clear to me.