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: https://github.com/kubeflow/pipelines/issues/11399

Checklist:

• [x] You have signed off your commits
• [x] 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[bot]]

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.

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[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. 🤷‍♂️ 🙂

[HumairAK]

/lgtm /approve

@tumido thank you! if you are up for it there are some other low hanging fruits here as well that can help with connecting against endpoints, I'm mainly thinking of ssl_ca_cert and verify_ssl if you're up for it 🥲

[google-oss-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: HumairAK

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Needs approval from an approver in each of these files: - ~~[sdk/OWNERS](https://github.com/kubeflow/pipelines/blob/master/sdk/OWNERS)~~ [HumairAK] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment