Use attrs/cattrs instead of custom serialization

[cat-state]

Depends on #306
This PR adds cattrs and attrs usage to remove us having to write custom ser/des code.

To keep supporting the ability to define MethodConfigs and *Trainers outside TRLX however, we need to keep some of the custom serialization/deserialization (rather than just making a Union of all the relevant config types and having cattrs deserialize it)

As a further improvement, we could make *TrainerConfig classes as we have for MethodConfig, and instantiate the trainers from a method on the deserialized *TrainerConfig, rather than a single TrainConfig with a stringly-typed name of the trainer class to instantiate. This would also let us give the different trainers different args as they need rather than getting trainer specific args from MethodConfig (like gen_kwargs)

[cat-state]

Converting to draft for discussion around refactoring and config breakage.