Should `SignRequest`'s `pubkey` be a `Credential` too?

#33 added support for adding certificates (thanks @baloo :bow: )

I wonder if [`SignRequest::pubkey`](https://github.com/wiktor-k/ssh-agent-lib/blob/main/src/proto/message/sign.rs#L16) should now be a `Credential` (which wraps keys and certs alike) since today it's just a `KeyData`.

Thoughts?

(I'll probably need to make a reproducer :thinking: )