[bdk_chain_redesign] Add apply_additions to IndexedTxGraph

* Get mutable index from `IndexedChainGraph`.
* Also add `apply_additions` method to `TxIndex` trait.

crates/chain/src/keychain/txout_index.rs

@@ -101,6 +101,10 @@ impl<K: Clone + Ord + Debug + 'static> TxIndex for KeychainTxOutIndex<K> {
         self.scan(tx)
     }
 
+    fn apply_additions(&mut self, additions: Self::Additions) {
+        self.apply_additions(additions)
+    }
+
     fn is_tx_relevant(&self, tx: &bitcoin::Transaction) -> bool {
         self.is_relevant(tx)
     }