Improve passing registries into the Ray workers

[tyler-griggs]

It is currently challenging to pass registries into Ray workers and validate their configuration. This is because registration must happen on a Ray worker for it to be usable during training, but config validation happens before calling into the Ray worker.

One option is to move config validation into the entrypoint of the head worker (e.g., skyrl_entrypoint). This would also let us do environment registration validation during config validation time.

[erictang000]

partially closed by #126 for algorithm support, but we should improve this for the SkyGym envs in a similar way (back the gym registry with named ray actor?)