feat(policy): adding return_extra to policy contracts

[Maximellerbach]

Preparing for more world models

Adding a return_extra kwargs to predict_action_chunk and select_action.
Main idea behind this is to prepare for different kind of models that might generate things other than actions at runtime.

Why an extra kwarg ?
-> the consumer decides wether he should have the extra data or not, for instance at inference it can be opted out.

The policy would have to implement something like:

if return_extra:
    extra = self._compute_extra(something)
    return action, extra
return action