Based on: https://issues.redhat.com/browse/RUN-2512

As a user, I would like to be able to remove all artifacts in the store with a single command.  The idea would be to add a `-all` or `-a` to the `rm` command which would be similar to containers, images, and other components of podman.

This is a feature request came from Yuhui Jiang (RedHat QE).

QE Suggests: 

1. Add several artifacts using `podman artifact add` commands.
2. Verify those added artifacts are visible with `podman artifact ls`
3. Execute `podman artifact rm -a`
4. Verify that no artifacts exist in the store with `podman artifact ls`