GnuTLS support for caching_sha2 authentication plugin

libgnutls: RSA-OAEP encryption scheme is now supported
To use it with an unrestricted RSA private key, one would need to
initialize a gnutls_x509_spki_t object with necessary parameters
for RSA-OAEP and attach it to the private key. It is also possible
to import restricted private keys if they are stored in PKCS#8
format