htools: add definitions for confd types

While we have some of these as plain types in Constants.hs, we add
proper ADT definitions for them in a new file. Furthermore, we add the
ConfdRequest and ConfdReply types here (in Python they are in
objects.py).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>