Add oauth authorization via an --access-token flag.

[oceanful]

Discovery of Application Default Credentials requires private keys to be distributed and discovered via the GOOGLE_APPLICATION_CREDENTIALS environment variable. This change allows kubemci to accept an oauth --access-token as an alternative.

[k8s-ci-robot]

Hi @oceanful. Thanks for your PR.

I'm waiting for a GoogleCloudPlatform or kubernetes 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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