ssl_cert_username_cea: Use CertificateExactAssertion as username

[minfrin]

Add the ssl_cert_username_cea parameter, which, when present, uses the CertificateExactAssertion as defined in RFC4523 appendix A.1 as the username.

[minfrin]

Quick ping on this one.

Keen to also contribute a ssl_cert_username_fingerprint=sha256 style option to match the functionality of postfix below, should I do this in the same PR or a new one?

https://www.postfix.org/postconf.5.html#permit_tls_clientcerts

[cmouse]

We'll try to come back to this soon.