feat(kubernetes_platform): Add k8s FieldPath as env to the kubernetes_platform

[Tomcli]

Description of your changes:
Part of #10155

Add new kubernetes_platform spec to set env from the metadata values and also the pod field values. The FieldPath value will always be a string based on the k8s documentation: https://kubernetes.io/docs/tasks/inject-data-application/environment-variable-expose-pod-information/

Checklist:

• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[Tomcli]

/assign @connor-mccarthy
/assign @chensun
cc @roytman

[google-oss-prow]

@Tomcli: 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	47a79f4	link	false	/test kfp-kubernetes-execution-tests

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[chensun]

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chensun

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