Feature - GetResourceHandler

Due to a recent flood of requests around `RespondWith` not being implemented, I've created this PR

`GetResourceHandler` replaces `RespondWith` that was previously part of `OnBeforeResourceLoad`. I've also cleaned up some code related to `RequestResponse`

- Split `RequestResponse` into a separate `Response` class and pass in `Request` as it's own param
- Implement `GetResourceHandler` which allows for providing a `Stream` for a file.
- Move `Response` into c# land