Liquid/Lightning drain (#553)

* Attempt drain using liquid or lightning payment

* Optimize estimation handling, use address in drain estimation

* Add drain option to PrepareSendRequest

* Block draining while there are pending payments

* Apply suggestions from code review

* Rename PayOnchainAmount to PayAmount

packages/dart/lib/src/model.dart

@@ -476,16 +476,16 @@ class OnchainPaymentLimitsResponse {
 }
 
 @freezed
-sealed class PayOnchainAmount with _$PayOnchainAmount {
-  const PayOnchainAmount._();
+sealed class PayAmount with _$PayAmount {
+  const PayAmount._();
 
   /// The amount in satoshi that will be received
-  const factory PayOnchainAmount.receiver({
+  const factory PayAmount.receiver({
     required BigInt amountSat,
-  }) = PayOnchainAmount_Receiver;
+  }) = PayAmount_Receiver;
 
   /// Indicates that all available funds should be sent
-  const factory PayOnchainAmount.drain() = PayOnchainAmount_Drain;
+  const factory PayAmount.drain() = PayAmount_Drain;
 }
 
 /// An argument when calling [crate::sdk::LiquidSdk::pay_onchain].
@@ -849,7 +849,7 @@ class PrepareLnUrlPayResponse {
 
 /// An argument when calling [crate::sdk::LiquidSdk::prepare_pay_onchain].
 class PreparePayOnchainRequest {
-  final PayOnchainAmount amount;
+  final PayAmount amount;
 
   /// The optional fee rate of the Bitcoin claim transaction in sat/vB. Defaults to the swapper estimated claim fee.
   final int? feeRateSatPerVbyte;
@@ -1005,16 +1005,16 @@ class PrepareSendRequest {
   final String destination;
 
   /// Should only be set when paying directly onchain or to a BIP21 URI
-  /// where no amount is specified
-  final BigInt? amountSat;
+  /// where no amount is specified, or when the caller wishes to drain
+  final PayAmount? amount;
 
   const PrepareSendRequest({
     required this.destination,
-    this.amountSat,
+    this.amount,
   });
 
   @override
-  int get hashCode => destination.hashCode ^ amountSat.hashCode;
+  int get hashCode => destination.hashCode ^ amount.hashCode;
 
   @override
   bool operator ==(Object other) =>
@@ -1022,7 +1022,7 @@ class PrepareSendRequest {
       other is PrepareSendRequest &&
           runtimeType == other.runtimeType &&
           destination == other.destination &&
-          amountSat == other.amountSat;
+          amount == other.amount;
 }
 
 /// Returned when calling [crate::sdk::LiquidSdk::prepare_send_payment].