Create a new `ROCSPStage6` feature flag which affects the behavior of
the SA. When enabled, this flag causes the `AddPrecertificate`,
`RevokeCertificate`, and `UpdateRevokedCertificate` methods to ignore
the OCSP response bytes provided by their caller. They will no longer
error out if those bytes are missing, and if the bytes are present they
will still not be written to the database.
This allows us to, in the future, cause the RA and CA to stop generating
those OCSP responses entirely, and stop providing them to the SA,
without causing any errors when we do.
Part of #6079
3a12177eab