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/esplora/reqwest.rs

@@ -117,6 +117,14 @@ impl GetTx for EsploraBlockchain {
     }
 }
 
+#[maybe_async]
+impl GetBlockHash for EsploraBlockchain {
+    fn get_block_hash(&self, height: u64) -> Result<BlockHash, Error> {
+        let block_header = await_or_block!(self.url_client._get_header(height as u32))?;
+        Ok(block_header.block_hash())
+    }
+}
+
 #[maybe_async]
 impl WalletSync for EsploraBlockchain {
     fn wallet_setup<D: BatchDatabase>(