DocumentReference.get() does not support optional arguments

[SHGoldfarb]

Reference:

python-mock-firestore/mockfirestore/document.py

Lines 66 to 67 in 0de34b1

	def get(self) -> DocumentSnapshot:
	return DocumentSnapshot(self, get_by_path(self._data, self._path))

The get method should support optional field_paths, transaction, retry and timeout arguments as documented here

get(
    field_paths: typing.Optional[typing.Iterable[str]] = None,
    transaction=None,
    retry: google.api_core.retry.retry_unary.Retry = _MethodDefault._DEFAULT_VALUE,
    timeout: typing.Optional[float] = None,
) -> google.cloud.firestore_v1.base_document.DocumentSnapshot