Specify the callback function to use in the server for retrieving
the secret hash of the username, realm and password for use in the
DIGEST-MD5 mechanism. The function can be later retrieved using
gsasl_server_callback_digest_md5_get().
DEPRECATED
This function is part of the old callback interface.
The new interface uses gsasl_callback_set() to set the application
callback, and uses gsasl_callback() or gsasl_property_get() to
invoke the callback for certain properties.