Groundwork to support OpenAI API endpoints that vLLM supports

[kfswain]

This PR sets up envoy to share request path attributes with EPP, and creates a set of maps to determine which routes we allow, and which are just passthrough.

Optionally, we can make the decision that EPP should do no route enforcement, and then we only map the RoutableEndpoints and assume any other route is just passthrough.

The route list was made from an intersection of the endpoints in:

• https://github.com/vllm-project/vllm/blob/main/vllm/entrypoints/openai/api_server.py
  &&
• https://platform.openai.com/docs/api-reference/introduction

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[hzxuzhonghu]

For these passthrough api sendpoints, users should setup policies to tell envoy not send the request to epp at all. It's better to shortcircuit in envoy.