Add cert support

[elsgaard]

Using SSH Certificates with simplessh

It is getting more popular to use certificates for ssh authentication, which i have added to simplessh.

In short, we need to parse the user’s private key and the certificate, create a signer using both the certificate and the private key, and use that as an auth method.

In order to keep this PR as small as possible, i have not added support for passphrases etc, but will follow in a separate PR