WIP: tls: Implement protocol version and ciphersuite name accessors

This allows the application to display protocol version and ciphersuite name.

However, there is a problem. It is not implemented correctly for GnuTLS, because GnuTLS only supports returning a ciphersuite name for TLS 1.2 and older ciphersuites. For TLS 1.3 we have to use gnutls_session_get_desc(), which provides much more information than just the ciphersuite name. So this commit is not correct, and we need to think harder about what we really want to do here.

The OpenSSL implementation works fine, though.