[Perf] Improve disaggregated prefill router performance

[YuhanLiu11]

This PR improves the router performance of disaggregated prefill. This PR creates the prefill_client and decode_client and sends the request accordingly to these clients.
This PR also makes the P and D instances able to use NVLink across GPUs.

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Thank you for your contribution to production-stack! Before submitting the pull request, please ensure the PR meets the following criteria. This helps us maintain the code quality and improve the efficiency of the review process.

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Please try to classify PRs for easy understanding of the type of changes. The PR title is prefixed appropriately to indicate the type of change. Please use one of the following:

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[ahinsutime]

It seems like endpoint argument is not used at the moment (so I think it can be removed safely).

[YuhanLiu11]

Fixed this! (endpoint is actually needed)

[zevisert]

Hey @YuhanLiu11 - the vllm deployment already has a securityContext defined on lines 163-166 of this file. I'm seeing errors with this chart in my deployment now because of mapping key "securityContext" already defined