feat: support extra payload data in transactions (vExtraData)

coinlib/lib/src/tx/sighash/legacy_signature_hasher.dart

@@ -69,6 +69,7 @@ final class LegacySignatureHasher extends SignatureHasher {
       inputs: modifiedInputs,
       outputs: modifiedOutputs,
       locktime: tx.locktime,
+      vExtraData: tx.vExtraData,
     );
 
     // Add sighash type onto the end

coinlib/lib/src/tx/transaction.dart

@@ -48,6 +48,7 @@ class Transaction with Writable {
   final List<Input> inputs;
   final List<Output> outputs;
   final Locktime locktime;
+  final Uint8List? vExtraData;
 
   /// Constructs a transaction with the given [inputs] and [outputs].
   /// [TransactionTooLarge] will be thrown if the resulting transction exceeds
@@ -60,6 +61,7 @@ class Transaction with Writable {
     required Iterable<Input> inputs,
     required Iterable<Output> outputs,
     this.locktime = Locktime.zero,
+    this.vExtraData,
   })  : inputs = List.unmodifiable(inputs),
         outputs = List.unmodifiable(outputs) {
     checkInt32(version);
@@ -184,13 +186,18 @@ class Transaction with Writable {
     }
 
     writer.writeUInt32(locktime.value);
+
+    if (vExtraData != null) {
+      writer.writeVarSlice(vExtraData!);
+    }
   }
 
   Transaction _newInputs(List<Input> newInputs) => Transaction(
         version: version,
         inputs: newInputs,
         outputs: outputs,
         locktime: locktime,
+        vExtraData: vExtraData,
       );
 
   T _requireInputOfType<T>(int inputN) {
@@ -348,6 +355,7 @@ class Transaction with Writable {
         ],
         outputs: outputs,
         locktime: locktime,
+        vExtraData: vExtraData,
       );
 
   /// Returns a new [Transaction] with the [output] added to the end of the
@@ -377,6 +385,7 @@ class Transaction with Writable {
       inputs: modifiedInputs,
       outputs: [...outputs, output],
       locktime: locktime,
+      vExtraData: vExtraData,
     );
   }
 
@@ -401,6 +410,7 @@ class Transaction with Writable {
           ),
           outputs: outputs,
           locktime: locktime,
+          vExtraData: vExtraData,
         )
       : this;