Add TxBuilder::drain_wallet

src/bdk.udl

@@ -144,6 +144,7 @@ interface TxBuilder {
   constructor();
   TxBuilder add_recipient(string address, u64 amount);
   TxBuilder fee_rate(float sat_per_vbyte);
+  TxBuilder drain_wallet();
   [Throws=BdkError]
   PartiallySignedBitcoinTransaction build([ByRef] Wallet wallet);
 };