Add doctrine readmodel support

I'm already using this in my projects. (Although I'm considering using only dbal)
Before I write unit tests I want to make sure you guys want to add this to this library.

Todo
- [x] check if this is something broadway wants to support
- [ ] write tests
- [ ] make it configurable in the bundle
well, the question is whether you want to enforce using the DB primary key as your model identifier. If yes, using ``find()`` is indeed much better for performance