Update pipeline_spec in oss

[rickyxie0929]

Description of your changes:

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

[google-oss-prow]

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[rickyxie0929]

/ok-to-test

[google-oss-prow]

@rickyxie0929: 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	a8d8677	link	false	/test kfp-kubernetes-execution-tests

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[gregsheremeta]

Could you elaborate in your commit messages and PR description as to what the purpose of this is? "Update pipeline_spec" is a little too generic. Perhaps describe what a Platform Spec is, and give some examples of what kinds of arbitrary configuration would go in the struct. Thanks!