[Core] Introduce env var RAY_LOG_TO_DRIVER

[jjyao]

Why are these changes needed?

Currently log_to_driver is an option of ray.init() but cannot be set through env var. This PR adds the ability to control this feature via env var as well.

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    method in Tune, I've added it in doc/source/tune/api/ under the
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  • This PR is not tested :(

[GeneDer]

LGTM! Thanks Jiajun!