Feature request: SerializersBuilder addAllBuilderFactories

When extending another packages' serializers you can `addAll` their serializers however I don't think there is a way to add all their BuilderFactories?

It would be great if you can do this:
```dart

import 'package:foobar/foobar.dart' as mypackage;
// ...
final Serializers serializers = (_$serializers.toBuilder()
..addAll(mypackage.serializers.serializers)
..addAllBuilderFactories(mypackage.serializers.builderFactories);
).build();
```

It would also be nice to addAllPlugins too but this would be less useful.