chain-api subsystem: implement BlockHeader messsage (#1778)

* chain-api subsystem: implement BlockHeader messsage

* update the guide

polkadot/roadmap/implementers-guide/src/node/utility/chain-api.md

@@ -14,6 +14,7 @@ On receipt of `ChainApiMessage`, answer the request and provide the response to
 
 Currently, the following requests are supported:
 * Block hash to number
+* Block hash to header
 * Finalized block number to hash
 * Last finalized block number
 * Ancestors

polkadot/roadmap/implementers-guide/src/types/overseer-protocol.md

@@ -140,6 +140,9 @@ enum ChainApiMessage {
 	/// Get the block number by hash.
 	/// Returns `None` if a block with the given hash is not present in the db.
 	BlockNumber(Hash, ResponseChannel<Result<Option<BlockNumber>, Error>>),
+	/// Request the block header by hash.
+	/// Returns `None` if a block with the given hash is not present in the db.
+	BlockHeader(Hash, ResponseChannel<Result<Option<BlockHeader>, Error>>),
 	/// Get the finalized block hash by number.
 	/// Returns `None` if a block with the given number is not present in the db.
 	/// Note: the caller must ensure the block is finalized.