This function can be used to set a callback to retrieve the username and
password for client PSK authentication.
The callback's function form is:
int (*callback)(gnutls_session_t, char** username,
gnutls_datum_t* key);
The username and key->data must be allocated using gnutls_malloc().
username should be ASCII strings or UTF-8 strings prepared using
the "SASLprep" profile of "stringprep".
The callback function will be called once per handshake.
The callback function should return 0 on success.
-1 indicates an error.