Authenticate command - requires response processing.
'mechanism' specifies which authentication mechanism is to
be used - it must appear in <instance>.capabilities in the
form AUTH=<mechanism>.
'authobject' must be a callable object:
data = authobject(response)
It will be called to process server continuation responses; the
response argument it is passed will be a bytes. It should return bytes
data that will be base64 encoded and sent to the server. It should
return None if the client abort response '*' should be sent instead.