Get block hash by its height

Create blockchain::GetBlockHash trait
with a method to get block hash given
a block height. Then, implement this
trait for all backends (Electrum, RPC
, Esplora, CBF). Referenced in issue 603.

src/blockchain/rpc.rs

@@ -169,6 +169,12 @@ impl GetHeight for RpcBlockchain {
     }
 }
 
+impl GetBlockHash for RpcBlockchain {
+    fn get_block_hash(&self, height: u64) -> Result<BlockHash, Error> {
+        Ok(self.client.get_block_hash(height)?)
+    }
+}
+
 impl WalletSync for RpcBlockchain {
     fn wallet_setup<D: BatchDatabase>(
         &self,