Westend<>Rococo Headers Relay (#875)

* Add modules for Rococo<>Westend header sync

* Use mock Westend and Rococo finaltiy tx calls

* Add Westend<>Rococo variants to `init_bridge`

* Add Westend<>Rococo variants to `relay_headers`

* Simplify the Rococo and Westend signing params

* Add `submit_finality_proof` mock Call variant

* Add note to more closely match `initialize` Call variant

* Accidentally committed `cargo-expand`ed code 🤦

* Add `initialize` Call variant to Rococo mock

* Fix call enums.

* Add explainatory comment.

* clippy.

* Add issue number.

* De-duplicate metrics customisation.

* Add comments to Rococo/Westend runtimes.

* Add scale-encoding compatibility test.

* Fix tests.

* Clippy.

Co-authored-by: Tomasz Drwięga <tomasz@parity.io>

bridges/primitives/chain-rococo/Cargo.toml

@@ -10,6 +10,7 @@ license = "GPL-3.0-or-later WITH Classpath-exception-2.0"
 parity-scale-codec = { version = "2.0.0", default-features = false, features = ["derive"] }
 
 # Bridge Dependencies
+bp-header-chain = { path = "../header-chain", default-features = false }
 bp-messages = { path = "../messages", default-features = false }
 bp-polkadot-core = { path = "../polkadot-core", default-features = false }
 bp-runtime = { path = "../runtime", default-features = false }
@@ -23,6 +24,7 @@ sp-version = { git = "https://github.com/paritytech/substrate", branch = "master
 [features]
 default = ["std"]
 std = [
+	"bp-header-chain/std",
 	"bp-messages/std",
 	"bp-polkadot-core/std",
 	"bp-runtime/std",