Enhancement: Improve the `find` command

We've updated the `find` command to support multiple patterns.

`restic find` is now able to list the snapshots containing a specific tree
or blob, or even the snapshots that contain blobs belonging to a given pack.
A list of IDs can be given, as long as they all have the same type.

The command `find` can also display the pack IDs the blobs belong to, if
the `--show-pack-id` flag is provided.

https://github.com/restic/restic/issues/1777
https://github.com/restic/restic/pull/1780
