Add support for specifying env versions for hosted RL

[manveerxyz]

Note

Low Risk
Additive config parsing that only affects RL environment identifiers and the request payload sent to the API; low risk aside from potential edge cases when @ appears in an env id.

Overview
Hosted RL configs can now pin specific environment versions.

EnvConfig and EvalEnvConfig add an optional version field, accept owner/name@version in id via a Pydantic model_validator, and include version in the environment payload sent to RLClient.create_run (and eval environments) when provided.

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