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
given that Doctrine manages the repository creation itself, it would allow to put the class in an inconsistent state if the instantiation does not give the repository associated to this entity manager