feat(cli): expose existing_token client property

[tumido]

Description of your changes:

This PR exposes existing_token option of kfp.Client to the CLI.

Resolves: #11399

Checklist:

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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]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[google-oss-prow]

Hi @tumido. 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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[hbelmiro]

/lgtm
/ok-to-test

[tumido]

@hbelmiro can you please approve the CI to be run on this? It seems ok-to-test is not enough and some additional button clicking or keyboard smashing is still required. 🤷‍♂️ 🙂

[DharmitD]

@HumairAK could you approve the CI runs for this PR? Otherwise,
/lgtm