feat: Add `GgrsLocal`

[johanhelsing]

A rollback aware version of Bevy's Local system params.

I wanted to write systems with local resources in order to reduce boilerplate. And also to implement a rollback aware verison of EventReader. I needed something that behaves like Local<T> but rolls back when bevy_ggrs does.

This is a draft because:

• [ ] It doesn't correctly handles systems that don't mutate every frame
• [ ] It doesn't work for systems behind run conditions. It would be ok to not support this, but it should consistently panic in that case.
• [x] Make API public
• [ ] Use it in examples
• [ ] Needs docs

Putting it out here since I don't have time to work on it now. Perhaps someone else has.