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/header-chain/src/lib.rs

@@ -55,6 +55,22 @@ impl AuthoritySet {
 	}
 }
 
+/// Data required for initializing the bridge pallet.
+///
+/// The bridge needs to know where to start its sync from, and this provides that initial context.
+#[derive(Default, Encode, Decode, RuntimeDebug, PartialEq, Eq, Clone)]
+#[cfg_attr(feature = "std", derive(Serialize, Deserialize))]
+pub struct InitializationData<H: HeaderT> {
+	/// The header from which we should start syncing.
+	pub header: H,
+	/// The initial authorities of the pallet.
+	pub authority_list: AuthorityList,
+	/// The ID of the initial authority set.
+	pub set_id: SetId,
+	/// Should the pallet block transaction immediately after initialization.
+	pub is_halted: bool,
+}
+
 /// base trait for verifying transaction inclusion proofs.
 pub trait InclusionProofVerifier {
 	/// Transaction type.