Emit mortal transactions from relay (#1073)

* mortal relay transactions

* sign block hash for mortal transactions

* TransactionEraOf

* add explanation

* enable mortal transactions for Westend -> Millau headers relay

* clippy

* spellcheck

bridges/relays/bin-substrate/src/chains/westend_headers_to_millau.rs

@@ -63,6 +63,7 @@ impl SubstrateFinalitySyncPipeline for WestendFinalityToMillau {
 
 	fn make_submit_finality_proof_transaction(
 		&self,
+		era: bp_runtime::TransactionEraOf<Millau>,
 		transaction_nonce: <Millau as Chain>::Index,
 		header: WestendSyncHeader,
 		proof: GrandpaJustification<bp_westend::Header>,
@@ -77,6 +78,7 @@ impl SubstrateFinalitySyncPipeline for WestendFinalityToMillau {
 		let transaction = Millau::sign_transaction(
 			genesis_hash,
 			&self.finality_pipeline.target_sign,
+			era,
 			transaction_nonce,
 			call,
 		);