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 
`
        - --endpoint-selector
        - "app=vllm-llama3-8b-instruct"
        - --endpoint-target-ports
        - "8000"
`

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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

[elevran]

cursory review to provide initial feedback (realizing this is work in progress)
The main question I have (and might be worth mentioning in the PR description) is the need for a new abstraction/type (EndPointsPool). A naive/simple solution (which perhaps does not work...) would be to copy the selector and port array into a Go InferencePool object and use datastore.PoolSet() along with disabling the Pool notification/reconciliation so it does not overwrite with nil.
Hopefully the rest of the code should not care or depend on the pool's origin (from command line or the API server)

[capri-xiyue]

assign @ahg-g for early review.

[capri-xiyue]

assign @kfswain for early review

[elevran]

My general point is that having a canonical internal type that represents the pool is useful, and abstracts how the EPP was configured with the pool.

Do we actually need pool as a canonical type or concept?
If so, and given the current code base and future changes in #1838 - is it warranted to replace the current InferencePool Go with a new type?

Given this from #1838:

1. Orchestrator agnostic deployment mode: static configuration of endpoints with zero dependency on k8s as a control plane, which means the reconcilers are not started as well.
2. k8s-native deployment modes
   2.1) No dependency on Gateway API or extension CRDs, endpoints are discovered using a pod selector and port flags
   2.2) No dependency Gateway APIs, extension CRDs are used to enable a richer feature set for EPP, but the proxy is statically
   ...

The Inference Pool is not needed at all in (1) and all reconcilers should be disabled, option (2.1) works well with a "fake" pool that has the minimal attributes set and a no pool reconciler, and (2.2) or later use the full CRD.
I don't think the internal type improves or changes any of that.

Ultimately we deal with inference endpoints (BTW: agree with @kfswain comment that the term is not used consistently across and we should make it more explicit). The InferencePool is merely one way to define attributes of service discovery for relevant endpoints.

[ahg-g]

I think we are going back and forth on a non material issue; both paths are reasonable and one can't argue that the one in this PR is wrong or harmful; it is also an implementation detail that I recommend to empower contributors to make.

[ahg-g]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahg-g, capri-xiyue

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[ahg-g]

/retest

[ahg-g]

/lgtm