feat(forget)!: Make keep-* Options and add keep-none

[aawsome]

Breaking Change: The keep-options for forget are now real Options.

This also adds an additional check which enforces that at least one keep-* option is set and adds a new keep-none option which can be use as single valid keep- option (if you want to remove all snapshots).

closes https://github.com/rustic-rs/rustic/issues/1191