[core] add extra_args to RPCProcessRequest and MQLLMEngineClient

[akeshet]

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[njhill]

@akeshet I don't think this is needed since we now have extra_args in SamplingParams: #13300

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @akeshet.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork