refactor: Replace NamespacedName with EndPointKey for endpoint identification

[zetxqx]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

This PR refactors the EPP's internal endpoint identification mechanism by replacing the generic Kubernetes NamespacedName with a specialized plugin.EndPointKey. This new type explicitly
includes the endpoint's port, providing a more precise way to identify backends across the datastore, metrics, and scheduling framework.

The previous NamespacedName is misleading for supporting DataParallel where an inferencePool has multiple targetPorts, please see: #2699 (comment) for an example.

Key Changes:

• Type Definition: Introduced EndPointKey in pkg/epp/framework/interface/plugin which encapsulates Name, Namespace, and Port.
• Datalayer: Updated EndpointMetadata to use the new Key field, enabling better tracking of multi-port or non-pod endpoints in the future.
• Interface Refactoring: Updated Datastore, PodList, and Handle interfaces to consume and return EndPointKey instead of NamespacedName.

Which issue(s) this PR fixes:

Related to: #2699

Does this PR introduce a user-facing change?:

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: zetxqx
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[ahg-g]

/hold

I would wait a bit, there are a number of pending PRs that will conflict with this

[k8s-ci-robot]

@zetxqx: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-gateway-api-inference-extension-verify-main	5e166f4	link	true	/test pull-gateway-api-inference-extension-verify-main
pull-gateway-api-inference-extension-test-e2e-main	5e166f4	link	true	/test pull-gateway-api-inference-extension-test-e2e-main
pull-gateway-api-inference-extension-test-unit-main	5e166f4	link	true	/test pull-gateway-api-inference-extension-test-unit-main

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[k8s-ci-robot]

PR needs rebase.

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