Enhancement: Make the `dump` command support `zip` format

Previously, restic could dump the contents of a whole folder structure only
in the `tar` format. The `dump` command now has a new flag to change output
format to `zip`. Just pass `--archive zip` as an option to `restic dump`.

https://github.com/restic/restic/pull/2433
https://github.com/restic/restic/pull/3081
