feat(backend): Add support for platform specs on K8s native API

[mprahl]

Description of your changes:

This adds an optional spec.platformSpec field to the PipelineVersion CRD.

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[google-oss-prow]

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

[mprahl]

/cc @VaniHaripriya

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[VaniHaripriya]

/lgtm
The changes looks good to me and the tests provide good coverage.