Fix handling of ServerHello in TLS v1.3 to support TLS v1.2 when
downgrading.
Added support in client and server examples for using downgrade method:
wolfSSLv23_client_method_ex() or wolfSSLv23_server_method_ex().
Add tests, using downgrade version, of client or server downgrading from
TLS v1.3 to TLS v1.2.