fix(backend): Allow initializing the Kubernetes client with a kubeconfig

[mprahl]

Description of your changes:

This makes it so that the API server will first try the in-cluster configuration and fallback to a kubeconfig. This is helpful when the API server is running outside of the cluster (e.g. locally) so you don't need to create token files.

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[google-oss-prow]

Hi @mprahl. 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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[hbelmiro]

/lgtm
/ok-to-test

[DharmitD]

/lgtm

[HumairAK]

Much needed @mprahl ++ !

[HumairAK]

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: HumairAK

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