netcmd: add custom json encoder for object type fields

netcmd: add custom json encoder for object type fields

The custom JSONEncoder class is also capable of encoding Dn objects to
str, and any object that has a __json__ method.

The __json__ method is not an official dunder method, but this has
been used by other frameworks too (like Pyramid).

Signed-off-by: Rob van der Linde <rob@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Joseph Sutton <josephsutton@catalyst.net.nz>