Add support for TLS handshake with raw public keys
It is possible to perform a TLS handshake with raw public keys instead of X.509 or OpenPGP certificates via RFC7250. These can be used in situations where the keys are verified using some external mechanism, and thus don't require the whole PKI or even OpenPGP containers. For that the TLS handshake of GnuTLS has to be extended to support RFC7250.