feature request: expose X509 certificates via the agent

[martinpaljak]

As per https://datatracker.ietf.org/doc/html/rfc6187#section-2.1

Right now I'm doing it manually with an "overlay" agent, but having this available allows to re-assemble the "PKI token" on the other side of the agent much like PKCS#11 and do other tricks (see also: https://github.com/martinpaljak/java-ssh-provider/wiki). Signature format is the same, so it is just adding the certificate blob as another entry.