Implement Deref<Target=Client> for ElectrumBlockchain

As pointed out in bitcoindevkit/rust-electrum-client#58 (comment)
there was no way to keep using the client once it was given to BDK.

src/blockchain/electrum.rs

@@ -25,6 +25,7 @@
 //! ```
 
 use std::collections::{HashMap, HashSet};
+use std::ops::Deref;
 
 #[allow(unused_imports)]
 use log::{debug, error, info, trace};
@@ -79,6 +80,14 @@ impl Blockchain for ElectrumBlockchain {
     }
 }
 
+impl Deref for ElectrumBlockchain {
+    type Target = Client;
+
+    fn deref(&self) -> &Self::Target {
+        &self.client
+    }
+}
+
 impl StatelessBlockchain for ElectrumBlockchain {}
 
 impl GetHeight for ElectrumBlockchain {