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/any.rs

@@ -120,6 +120,13 @@ impl GetTx for AnyBlockchain {
     }
 }
 
+#[maybe_async]
+impl GetBlockHash for AnyBlockchain {
+    fn get_block_hash(&self, height: u64) -> Result<BlockHash, Error> {
+        maybe_await!(impl_inner_method!(self, get_block_hash, height))
+    }
+}
+
 #[maybe_async]
 impl WalletSync for AnyBlockchain {
     fn wallet_sync<D: BatchDatabase>(