This function can be used to set a callback to retrieve the user's PSK credentials.
The callback's function form is:
int (*callback)(gnutls_session_t, const char* username,
gnutls_datum_t* key);
username contains the actual username.
The key must be filled in using the gnutls_malloc().
In case the callback returned a negative number then gnutls will
assume that the username does not exist.
The callback function will only be called once per handshake. The
callback function should return 0 on success, while -1 indicates
an error.