Add a command to purge the relay chain only (#306)

* Add a command to purge the relay chain only

* WIP

* Update rococo-parachains/src/cli.rs

Co-authored-by: Bastian Köcher <[email protected]>

* Move cli stuff to its own crate

* Copyright dates

* Test not working for some reason...

* WIP

* Revert "WIP"

This reverts commit f97cd63742c7df822e4a6e52a29db5e0f56b7bfa.

* Fix test to use provided relay chain

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <[email protected]>

* Add hint about which database could not be purged

Co-authored-by: Bastian Köcher <[email protected]>