Using the CertBuilder, it is hard to add data to selfsigs.
CertBuilder, it is easy to add a User ID using
[CertBuilder::add_userid](https://docs.sequoia-pgp.org/sequoia_openpgp/cert/struct.CertBuilder.html#method.add_userid). Unfortunately, the self-signature can't be customized. But, it shouldn't be hard to add a variant of
CertBuilder::add_userid, which takes a
CertBuilder::build is called, it just creates a fresh
SignatureBuilder and sets a few values. It could just as well using an existing
SignatureBuilder and tweak it.