introduce a method for fetching relay chain header to RelayChainInterface (#2794)

* introduce a method for fetching header to RelayChainInterface

* cargo fmt

* return Option<Header>

cumulus/client/relay-chain-interface/src/lib.rs

@@ -26,6 +26,7 @@ use jsonrpsee_core::Error as JsonRpcError;
 use parity_scale_codec::Error as CodecError;
 use sp_api::ApiError;
 
+use cumulus_primitives_core::relay_chain::BlockId;
 pub use cumulus_primitives_core::{
 	relay_chain::{
 		CommittedCandidateReceipt, Hash as PHash, Header as PHeader, InboundHrmpMessage,
@@ -110,6 +111,9 @@ pub trait RelayChainInterface: Send + Sync {
 	/// Get the hash of the current best block.
 	async fn best_block_hash(&self) -> RelayChainResult<PHash>;
 
+	/// Fetch the block header of a given height
+	async fn header(&self, block_id: BlockId) -> RelayChainResult<Option<PHeader>>;
+
 	/// Get the hash of the finalized block.
 	async fn finalized_block_hash(&self) -> RelayChainResult<PHash>;
 
@@ -293,4 +297,8 @@ where
 	) -> RelayChainResult<Pin<Box<dyn Stream<Item = PHeader> + Send>>> {
 		(**self).new_best_notification_stream().await
 	}
+
+	async fn header(&self, block_id: BlockId) -> RelayChainResult<Option<PHeader>> {
+		(**self).header(block_id).await
+	}
 }