[feat] Latency Prediction CI tests

[Gregory-Pereira]

What type of PR is this?
Add one of the following kinds:
/kind feature
/area conformance-test

What this PR does / why we need it:

Functional / regression testing for latency prediction. Provides maintainers more clarity about if a change is affecting the codebase.

How it works:
On open PRs that modify the latencyprediction directory it will rebuild the images, tag them both by sha- and tag them by pr number. It will push those ephemeral images to this repo's package registry in ghcr.io (open to alternatives here, I know y'all do something else with prow for on push builds). Then it patches the new images into the kustomization.yaml. It will then roll our functional tests with these new images.

Important Technical Detail
After running these tests a bunch of times, I realized when you deploy the job at the same time as prediction and training servers, it will automatically skip every fixture in the job, which will show up as a pass. Instead we need to rollout the prediction and training servers, waiting for them to come online, before creating our test job.

Why only run this on open PRs and not on pushes to main (merged work)?
This is because the cloudbuild.yaml will already build images for every push to main - so if we want to test merged work too we should refactor this to a workflow call.

Does this PR introduce a user-facing change?:
No user facing changes

cc @kaushikmitr @kfswain

[k8s-ci-robot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Gregory-Pereira
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[kaushikmitr]

this looks good to me. will catch regressions in the latencypredictor python module. @Gregory-Pereira is this WIP?

[ahg-g]

needs to be opened against the llm-d repo after the move.