Adds support for returning a custom header from validate_block (#825)

* Adds support for returning a custom header from `validate_block`

This adds support for returning a custom header from `validate_block`. Before this, we always
returned the header of the block that was validated (and still do it by default). However, after
this pr it is possible to set a custom header or better custom head data that will be returned
instead from `validate_block`. This can be for example when a chain wants to fork.

* FMT

cumulus/test/service/src/chain_spec.rs

@@ -128,5 +128,6 @@ fn testnet_genesis(
 			balances: endowed_accounts.iter().cloned().map(|k| (k, 1 << 60)).collect(),
 		},
 		sudo: cumulus_test_runtime::SudoConfig { key: root_key },
+		transaction_payment: Default::default(),
 	}
 }