refactor(backend): rename api package to v1beta1

[Linchin]

Description of your changes:

• Refactored backend api to v1beta1, and changed backend imports accordingly
• Updated scripts generating api clients (except python client), which generates clients based on api version
• Generated v2beta1 clients, but not yet used

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.

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[google-oss-prow[bot]]

@Linchin: 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
kubeflow-pipeline-e2e-test	d58659a67305a9f41288413d2b870bf01ae0d180	link	true	/test kubeflow-pipeline-e2e-test

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[chensun]

/hold

@Linchin Let's get consent on design first before moving forward with this change.