[KEP-3104] Update KEP with credential plugin allowlist

[pmengelbert]

• One-line PR description: Update kuberc KEP with credential plugin allowlist

• Issue link: Separate kubectl user preferences from cluster configs #3104

• Other comments:

Currently, the kubeconfig may specify arbitrary binaries to run as
client-go credential plugins. Due to the fact that kubeconfig files are
often generated, and because they contain a lot of noise, there is
significant friction in manually inspecting the kubeconfig for
suspicious binaries after it is generated.

To encourage secure behavior, we want to introduce an allowlist to the
kuberc, which will describe the conditions under which a binary plugin
may be run. Currently the only condition is the name (absolute path or
basename of a binary found in PATH).

[linux-foundation-easycla]

• ✅ login: enj / name: Mo Khan (623d7eb)
• ❌ - login: @pmengelbert / name: Peter Engelbert . The commit (1496033, ce844ec, f8ed9ae, 692b82e) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[k8s-ci-robot]

Hi @pmengelbert. Thanks for your PR.

I'm waiting for a 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.

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[enj]

/sig auth

[enj]

This LGTM from a SIG Auth perspective (I added us as a participating SIG).

[enj]

/ok-to-test