Accept client certificates from an authn/authz plugin

[greed42]

(Plugin interface reference: https://kubernetes.io/docs/reference/access-authn-authz/authentication/#input-and-output-formats)

When handling the response from the authn/authz plugin, token will be used if provided, which maintains current behaviour. Newly added is handling clientCertificateData: if it is present, that certificate (and its key) will be used as provided by the plugin. (And any certificate/key pair provided via the users section of the configuration file will be ignored.)

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[roycaihw]

/lgtm
/approve

Thanks!

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This pull-request has been approved by: greed42, roycaihw

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