Allow freezing/unfreezing config

> Are we currently mandating that the config be read-only after overrides are applied? If not, can that be added?

We currently have places where we modify the configs: https://github.com/NVIDIA/reinforcer/blob/main/examples/run_grpo_math.py#L255

how should we deal with this sort of thing? It's not actually obvious to me if we should hard prevent people from using the configs for global variable passing for minor things like this.

_Originally posted by @SahilJain314 in https://github.com/NVIDIA/reinforcer/issues/80#issuecomment-2767095183_