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"
`

[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

[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

[ahg-g]

there is no existing library for that in k8s?

[capri-xiyue]

I didn't find existing library in k8s to change a int[] string to []int. As this is not like selector, probably it makes sense that k8s doesn't have it natively

[ahg-g]

why do we need this?

[capri-xiyue]

To avoid a giant PR, I kind of keep the metrics collector part unchanged in this PR.
Previously the metrics collector need inferencepool name, see https://github.com/kubernetes-sigs/gateway-api-inference-extension/blob/main/pkg/epp/metrics/collectors/inference_pool.go#L76.

As in EPP standalone mode, there is no inferencepool, I use the EPP name here.

Later I will revisit it to see whether there is better way to do it. But I think combing that in this then this PR will be too large.

[ahg-g]

metadata.name is the name of the pod, which will look odd as it will include a hash at the end, like my-epp-xyz-abc if the deployment name was my-epp, perhaps extract the deployment name in code by dropping the hash suffix?