Add TxBuilder::drain_to

src/bdk.udl

@@ -145,6 +145,7 @@ interface TxBuilder {
   TxBuilder add_recipient(string address, u64 amount);
   TxBuilder fee_rate(float sat_per_vbyte);
   TxBuilder drain_wallet();
+  TxBuilder drain_to(string address);
   [Throws=BdkError]
   PartiallySignedBitcoinTransaction build([ByRef] Wallet wallet);
 };