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 <bkchr@users.noreply.github.com>

* 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 <bkchr@users.noreply.github.com>

* Add hint about which database could not be purged

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

cumulus/rococo-parachains/Cargo.toml

@@ -56,6 +56,7 @@ sp-offchain = { git = "https://github.com/paritytech/substrate", branch = "maste
 jsonrpc-core = "15.1.0"
 
 # Cumulus dependencies
+cumulus-cli = { path = "../cli" }
 cumulus-client-consensus-relay-chain = { path = "../client/consensus/relay-chain" }
 cumulus-client-collator = { path = "../client/collator" }
 cumulus-client-service = { path = "../client/service" }
@@ -76,6 +77,7 @@ substrate-build-script-utils = { git = "https://github.com/paritytech/substrate"
 assert_cmd = "0.12"
 nix = "0.17"
 rand = "0.7.3"
+tempfile = "3.2.0"
 tokio = { version = "0.2.13", features = ["macros"] }
 
 # Polkadot dependencies