prepare-pay-onchain: add option for drain (#464)

* prepare-pay-onchain: add option for drain in req

* Fix clippy

* ChainSwapStateHandler: gracefully handle building both drain and non-drain lockups

* Send Chain swap: use standard feerate when estimating lockup tx fee

* UDL: move new drain field above the last PreparePayOnchainRequest optional field

* UDL: move new drain field optional

* prepare-pay-onchain: treat normal payment as drain if receiver amount is high enough

If the receiver amount is as high as it would be in case of drain, treat the current prepare-pay-onchain as drain, even if the drain flag is not set.

* build_drain_tx: add optional amount validation

* Add PayOnchainAmount enum to cover amount types (drain, receiver)

* Add ability to find max_receiver_amount_sat for non-drain sends

* Revert "Add ability to find max_receiver_amount_sat for non-drain sends"

This reverts commit 60ee1c768021810f72bc64a8ada69d35b638185e.

* prepare_pay_onchain: treat drain and non-drain cases separately

If the non-drain case is chosen with a receiver_amount equivalent to what drain would have calculated, it results in an error. For drain, the caller has to explicitly choose PayOnchainAmount::Drain.

* CLI: send-onchain-payment accepts optional amount

* CLI: add docs for send-onchain-payment drain arg

* SDK: expand docs for prepare_pay_onchain

* Re-generate RN bindings

* Re-generate flutter bindings

packages/dart/lib/src/model.dart

@@ -405,6 +405,19 @@ class OnchainPaymentLimitsResponse {
           receive == other.receive;
 }
 
+@freezed
+sealed class PayOnchainAmount with _$PayOnchainAmount {
+  const PayOnchainAmount._();
+
+  /// The amount in satoshi that will be received
+  const factory PayOnchainAmount.receiver({
+    required BigInt amountSat,
+  }) = PayOnchainAmount_Receiver;
+
+  /// Indicates that all available funds should be sent
+  const factory PayOnchainAmount.drain() = PayOnchainAmount_Drain;
+}
+
 /// An argument when calling [crate::sdk::LiquidSdk::pay_onchain].
 class PayOnchainRequest {
   final String address;
@@ -695,23 +708,25 @@ class PrepareBuyBitcoinResponse {
 
 /// An argument when calling [crate::sdk::LiquidSdk::prepare_pay_onchain].
 class PreparePayOnchainRequest {
-  final BigInt receiverAmountSat;
+  final PayOnchainAmount amount;
+
+  /// The optional fee rate of the Bitcoin claim transaction. Defaults to the swapper estimated claim fee.
   final int? satPerVbyte;
 
   const PreparePayOnchainRequest({
-    required this.receiverAmountSat,
+    required this.amount,
     this.satPerVbyte,
   });
 
   @override
-  int get hashCode => receiverAmountSat.hashCode ^ satPerVbyte.hashCode;
+  int get hashCode => amount.hashCode ^ satPerVbyte.hashCode;
 
   @override
   bool operator ==(Object other) =>
       identical(this, other) ||
       other is PreparePayOnchainRequest &&
           runtimeType == other.runtimeType &&
-          receiverAmountSat == other.receiverAmountSat &&
+          amount == other.amount &&
           satPerVbyte == other.satPerVbyte;
 }