Enhancement: find: Add options for searching IDs (blobs, trees...)

We've enhanced the `find` command to be 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