Enhancement: Add `--compress` flag to `dump` command to compress archive

Restic did not compress the archives that was created by using the
`dump` command. It now allows to save some disk space when exporting
archives by adding a `--compress` flag. The DEFLATE algorithm is used
for "zip" archives, and the gzip algorithm for "tar" archives,
resulting in a .tar.gz or .tgz file. Not compressing the archive
is still the default.

https://github.com/restic/restic/pull/5054