Rename PodInfo struct to EndpointMetadata to better reflect its purpose

[shmuelk]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
Gateway Inference Extension (GIE) PR #1663 added support for vLLM's Data Parallel feature. This feature enables running more than one exposed vLLM HTTP endpoint in a pod/container.

Previously the GIE assumed that there was only one vLLM endpoint per pod. Thus it tracked per pod information and called it PodInfo.

To simplify things PR #1663 simply treated each of the vLLM endpoints, described in a GIE InferencePool, as another pod with an appropriate PodInfo struct.

It was pointed out during the review of PR #1663 by @kfswain that we need to rename some of the structs due to the changes. Details can be found here

This PR is the first of several that will refactor/rename various structs/APIs to better reflect their usage.

In particular this PR:

• Renames PodInfo (in pkg/epp/datalayer) to EndpointMetadata
• Renames EndpointPodState (in pkg/epp/datalayer) to EndpointMetaState
  • Its functions GetPod() and UpdatePod() have been renamed GetMetadata() and UpdateMetadata() repectively
• Many places in the code that referenced backendmetrics.PodMetrics now referrece datalayer.Endpoint directly

Future PRs will do more of this refactoring/renaming and will improve variable names in the code as well.

None

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