Enhancement: Add command for copying snapshots between repositories

We've added a copy command, allowing you to copy snapshots from one
repository to another.

Note that this process will have to read (download) and write (upload) the
entire snapshot(s) due to the different encryption keys used on the source
and destination repository. Also, the transferred files are not re-chunked,
which may break deduplication between files already stored in the
destination repo and files copied there using this command.

https://github.com/restic/restic/issues/323
https://github.com/restic/restic/pull/2606