csr: add expirationSeconds field to control cert lifetime
csr: add expirationSeconds field to control cert lifetime

This change updates the CSR API to add a new, optional field called
expirationSeconds.  This field is a request to the signer for the
maximum duration the client wishes the cert to have.  The signer is
free to ignore this request based on its own internal policy.  The
signers built-in to KCM will honor this field if it is not set to a
value greater than --cluster-signing-duration.  The minimum allowed
value for this field is 600 seconds (ten minutes).

This change will help enforce safer durations for certificates in
the Kube ecosystem and will help related projects such as
cert-manager with their migration to the Kube CSR API.

Future enhancements may update the Kubelet to take advantage of this
field when it is configured in a way that can tolerate shorter
certificate lifespans with regular rotation.

Signed-off-by: Monis Khan <mok@vmware.com>

Kubernetes-commit: cd91e59f7c351fce47c064a5162c2cb79075159c

kubernetes/kubectl,kubernetes/kubectl