Rename part two

[shmuelk]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
This PR is a second PR in a set of PRs that is attempting to cleanup the code base WRT to Pods vs Model Server Endpoints.

It does this in two ways:

1. Changes various types, function names, field names, and variables to better reflect that they are related to either and endpoint or an endpoint's metadata.
2. Removes the aliasing of backend.Pod to datalayer.EndpointMetadata

As Model Servers evolve, vLLM or others, one can not assume that there is a one to one relationship between pods and Model Servers. That is a pod may contain multiple Model Servers each. on its own port, such as vLLM's Data Parallel support. On the other hand Model Servers may do there own internal multi-pod routing as well.

Does this PR introduce a user-facing change?:

NONE

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: shmuelk
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[k8s-ci-robot]

PR needs rebase.

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