feat: add get_variant_field() method

[knopki]

Add get_varian_field() method so we can get variant field by handle:

>>> import insales
>>> insales_api = insales.InSalesApi.from_credentials(
...     'XXX',
...     'XXX',
...     'XXX',
...     retry_on_503=True,
...     retry_on_socket_error=True,
... )
>>> 
>>> var = insales_api.get_variant_field('quantity_msk1')
>>> print(var)
{'id': 6948, 'application-id': None, 'type': 'VariantField::TextField', 'handle': 'quantity_msk1', 'position': 1, 'is-hidden': False, 'title': 'Остаток в офисе'}
>>>