Enhancement: Support forgetting all snapshots

The `forget` command now supports the `--unsafe-allow-remove-all` option, which
removes all snapshots in the repository.

This option must always be combined with a snapshot filter (by host, path or
tag). For example, the command `forget --tag example --unsafe-allow-remove-all`
removes all snapshots with the tag "example".

https://github.com/restic/restic/pull/4764
