Find all gRPC fields which represent DNS Names -- sometimes called
"identifier", "hostname", "domain", "identifierValue", or other things
-- and unify their naming. This naming makes it very clear that these
values are strings which may be included in the SAN extension of a
certificate with type dnsName.
As we move towards issuing IP Address certificates, all of these fields
will need to be replaced by fields which carry both an identifier type
and value, not just a single name. This unified naming makes it very
clear which messages and methods need to be updated to support
non-dnsName identifiers.
Part of https://github.com/letsencrypt/boulder/issues/7647