This function sets a callback to be called in order to retrieve the certificate
to be used in the handshake.
The callback's function prototype is:
int (*callback)(gnutls_session_t, gnutls_retr_st* st);
st should contain the certificates and private keys.
If the callback function is provided then gnutls will call it, in the
handshake, after the certificate request message has been received.
The callback function should set the certificate list to be sent, and
return 0 on success. The value (-1) indicates error and the handshake
will be terminated.