Certificate improvements

This series adds support for ECDSA and ED25519 certificates. It also modifies the certificate implementation to allow them to be used in signature verification. This enables certificate authentication to libssh servers that choose to support it.

Aside from parsing public keys out of certificates, this series does no addition certificate parsing. This means that a user wanting to support certificate authentication in their server application would need to find some way to parse the certificate on their own. Assuming I find the time, I'd love to add tools for processing certificates in another series.