[WIP] Enable EPP to support endpoint discovery using pod selector

[capri-xiyue]

What type of PR is this?

/kind feature

What this PR does / why we need it:
See #1779
Which issue(s) this PR fixes:

Part of #1779

Does this PR introduce a user-facing change?:

NONE to existing features with inferencepool. 
But users can use epp without inference pool via args 
`
        - --selector
        - "app:vllm-llama3-8b-instruct"
        - --target-ports
        - "8000"
`

[k8s-ci-robot]

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[capri-xiyue]

No need to review it right now. I just made the CUJ of standalone epp work without inferencepool. Still need to fix the e2e and ut