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-westend/src/lib.rs

@@ -21,6 +21,7 @@
 #![allow(clippy::unnecessary_mut_passed)]
 
 use bp_messages::{LaneId, MessageNonce, UnrewardedRelayersState, Weight};
+use bp_runtime::Chain;
 use sp_std::prelude::*;
 use sp_version::RuntimeVersion;
 
@@ -42,9 +43,33 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	transaction_version: 5,
 };
 
+/// Westend Runtime `Call` enum.
+///
+/// The enum represents a subset of possible `Call`s we can send to Westend chain.
+/// Ideally this code would be auto-generated from Metadata, because we want to
+/// avoid depending directly on the ENTIRE runtime just to get the encoding of `Dispatchable`s.
+///
+/// All entries here (like pretty much in the entire file) must be kept in sync with Westend
+/// `construct_runtime`, so that we maintain SCALE-compatibility.
+///
+/// See: https://github.com/paritytech/polkadot/blob/master/runtime/westend/src/lib.rs
 #[derive(parity_scale_codec::Encode, parity_scale_codec::Decode, Debug, PartialEq, Eq, Clone)]
 pub enum Call {
-	MockModule,
+	/// Rococo bridge pallet.
+	#[codec(index = 40)]
+	BridgeGrandpaRococo(BridgeGrandpaRococoCall),
+}
+
+#[derive(parity_scale_codec::Encode, parity_scale_codec::Decode, Debug, PartialEq, Eq, Clone)]
+#[allow(non_camel_case_types)]
+pub enum BridgeGrandpaRococoCall {
+	#[codec(index = 0)]
+	submit_finality_proof(
+		<PolkadotLike as Chain>::Header,
+		bp_header_chain::justification::GrandpaJustification<<PolkadotLike as Chain>::Header>,
+	),
+	#[codec(index = 1)]
+	initialize(bp_header_chain::InitializationData<<PolkadotLike as Chain>::Header>),
 }
 
 impl sp_runtime::traits::Dispatchable for Call {