Make NIXL backend as an implementation of base backend.

[jokerwyt]

Extended #5006

Not ready yet. Things to re-consider:

• Handshake and bootstrap workflow for xPyD, TP>8.
• What extra server args do we need? And how to align with other backend?
• What extra request field do we need? Remember to pass them through in all API endpoints. And how to align with other backend?
• Fix mini_lb on /v1/chat/completions.
• Remove all debug print.

Motivation

Modifications

Checklist

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[jokerwyt]

The diff is too large to merge. I will take care of it after PD interface is stable.