expose v3 TransformRounds for aes kdf.

lib/src/kdbx_format.dart

@@ -698,8 +698,7 @@ class KdbxFormat {
 
   static Future<Uint8List> _generateMasterKeyV3(
       KdbxHeader header, Credentials credentials) async {
-    final rounds = ReaderHelper.singleUint64(
-        header.fields[HeaderFields.TransformRounds].bytes);
+    final rounds = header.v3KdfTransformRounds;
     final seed = header.fields[HeaderFields.TransformSeed].bytes;
     final masterSeed = header.fields[HeaderFields.MasterSeed].bytes;
     _logger.finer(

lib/src/kdbx_header.dart

@@ -524,6 +524,9 @@ class KdbxHeader {
   VarDictionary get readKdfParameters => VarDictionary.read(
       ReaderHelper(fields[HeaderFields.KdfParameters].bytes));
 
+  int get v3KdfTransformRounds =>
+      ReaderHelper.singleUint64(fields[HeaderFields.TransformRounds].bytes);
+
   void writeKdfParameters(VarDictionary kdfParameters) =>
       _setHeaderField(HeaderFields.KdfParameters, kdfParameters.write());