Fix cable serialize

[srfwx]

Fixes: #555

The way pynetbox currently handles mixed lists doesn't allow to re-instantiate the proper nested object types when loading values from a serialized() object. This is because the generic_list_parser "gobbles" that information by construction specific Record types from the mixed list.

Example:

>>> print(cable.serialize())

{'a_terminations': [530082],
 'b_terminations': [314260],
 'color': '',
 'comments': '',
 'created': '2022-05-03T02:00:00+02:00',
 'custom_fields': {},
 'description': '',
 'display': '1001',
 'id': 155652,
 'label': '1001',
 'last_updated': '2022-06-29T14:36:49.386886+02:00',
 'length': None,
 'length_unit': None,
 'status': 'connected',
 'tags': [],
 'tenant': 2510,
 'type': '',
 'url': 'https://localhost:8000/api/dcim/cables/155652/'}

Patching the record a_terminations / b_terminations with new ID wouldn't work as with other nested Record types as in this case the API expects a dict with object_type and object_id keys.

I propose to solve this by introducing a GenericListItem class that more accurately stores the state of the API response, while allowing a nicer serialization

>>> print(cable.serialize())

 {'a_terminations': [{'object_id': 530082, 'object_type': 'dcim.interface'}],
 'b_terminations': [{'object_id': 314260, 'object_type': 'dcim.frontport'}],
 'color': '',
 'comments': '',
 'created': '2022-05-03T02:00:00+02:00',
 'custom_fields': {},
 'description': '',
 'display': '1001',
 'id': 155652,
 'label': '1001',
 'last_updated': '2022-06-29T14:36:49.386886+02:00',
 'length': None,
 'length_unit': None,
 'status': 'connected',
 'tags': [],
 'tenant': 2510,
 'type': '',
 'url': 'https://localhost:8000/api/dcim/cables/155652/'}

The __getattr__ on the new class, allows to maintain backward compatibility to some extent.

>>> print(cables.a_terminations[0].label)
'ETH-1'

This involves a change in type of objects returned in mixed/generic lists now always being GenericListItem