feat(kubernetes_platform): Update kubernetes_platform go package to include imagePullPolicy.

[revit13]

Description of your changes:
Part of #9714 and #9980

Update kubernetes_platform go package to include image pull policy. Once this is ready, I will also update the backend driver, compiler, and SDK with this new feature.

Checklist:

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[google-oss-prow]

Hi @revit13. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[Tomcli]

/ok-to-test

[zijianjoy]

/assign @Tomcli

/cc @chensun

[Tomcli]

/lgtm

[Tomcli]

/lgtm

/assign @connor-mccarthy

[chensun]

/lgtm

[chensun]

nit: if this is a stable value set, maybe we can make this an enum instead of string? (I don't have strong opinion on this, feel free to ignore)

[revit13]

@chensun I think I'll leave it as a string if that's ok. Also, it seems that in Kubernetes it is defined as a string as well, but I am not sure if it's the same case. Anyway, in the second part of the PR there is a check for the values. Thanks

[chensun]

/approve

[google-oss-prow]

@revit13: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
kfp-kubernetes-execution-tests	921a7e8	link	false	/test kfp-kubernetes-execution-tests

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[Tomcli]

/lgtm

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chensun, Tomcli

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