Support client side ephemeral certificates

Add support for sending ephemeral certificate fingerprint digest to client. More details in https://mariadb.org/mission-impossible-zero-configuration-ssl

If a MaxScale listener has `ssl=1` but both `ssl_key` and `ssl_cert` are unset, MaxScale should generate the key and certificate during startup and keep it stored in memory. The certificate is used when communicating with clients. If client accepts the certificate (no clients currently support this) and sends correct password, MaxScale then sends a hash of the password and certificate to the client to prove that it knows the client's password hash. If client password is empty, MaxScale should not send the hash.