Add a new gRPC method `UpdateRevokedCertificate` to the SA. This
method takes the same argument as the existing `RevokeCertificate` RPC,
but only operates on certificates that have already been revoked with a
reason other than keyCompromise (c.f. `RevokeCertificate`, which only
operates on certificates that have not been revoked).
One thing to be careful of here is that storing an updated revocation reason
should not also change the revocation date. To support this, add a new field
to the existing `RevokeCertificateRequest` that allows us to differentiate the
time at which the new OCSP response was created, and the time at which
the revocation went into effect.
Part of #5936
745e69e7f9