Add OnlineWallet::getBalance()

src/bdk.udl

@@ -101,4 +101,6 @@ interface OnlineWallet {
     Network get_network();
     [Throws=BdkError]
     void sync(BdkProgress progress_update, u32? max_address_param);
+    [Throws=BdkError]
+    u64 get_balance();
 };

src/lib.rs

@@ -149,6 +149,10 @@ impl OnlineWallet {
             .unwrap()
             .sync(BdkProgressHolder { progress_update }, max_address_param)
     }
+
+    fn get_balance(&self) -> Result<u64, Error> {
+        self.wallet.lock().unwrap().get_balance()
+    }
 }
 
 uniffi::deps::static_assertions::assert_impl_all!(OfflineWallet: Sync, Send);