fix: split to packages to support gql gen

packages/gql_github/lib/utils/date_time_serializer.dart

@@ -0,0 +1,28 @@
+import 'package:built_value/serializer.dart';
+
+class DateTimeSerializer implements PrimitiveSerializer<DateTime> {
+  @override
+  DateTime deserialize(
+    Serializers serializers,
+    Object serialized, {
+    FullType specifiedType = FullType.unspecified,
+  }) {
+    assert(serialized is String,
+        "DateTimeSerializer expected 'String' but got ${serialized.runtimeType}");
+    return DateTime.parse(serialized as String);
+  }
+
+  @override
+  Object serialize(
+    Serializers serializers,
+    DateTime dt, {
+    FullType specifiedType = FullType.unspecified,
+  }) =>
+      dt.toString();
+
+  @override
+  Iterable<Type> get types => [DateTime];
+
+  @override
+  String get wireName => 'DateTime';
+}