Support filter kw on collections.where

[ingwinlu]

google-cloud-firestore = "^2.11.1"
mock-firestore = "^0.11.0"

site-packages/google/cloud/firestore_v1/base_collection.py:290: UserWarning: Detected filter using positional arguments. Prefer using the 'filter' keyword argument instead.

i.e. instead of
col.where(_DELETED_ATTR, "==", False)

it is recommended to:

_DELETED_FILTER = FieldFilter(_DELETED_ATTR, "==", False)
col = col.where(filter=_DELETED_FILTER)

current where typesig + docs:

    def where(
        self,
        field_path: Optional[str] = None,
        op_string: Optional[str] = None,
        value=None,
        *,
        filter=None
    ) -> BaseQuery:
        """Create a "where" query with this collection as parent.

        See
        :meth:`~google.cloud.firestore_v1.query.Query.where` for
        more information on this method.

        Args:
            field_path (str): A field path (``.``-delimited list of
                field names) for the field to filter on. Optional.
            op_string (str): A comparison operation in the form of a string.
                Acceptable values are ``<``, ``<=``, ``==``, ``>=``, ``>``,
                and ``in``. Optional.
            value (Any): The value to compare the field against in the filter.
                If ``value`` is :data:`None` or a NaN, then ``==`` is the only
                allowed operation.  If ``op_string`` is ``in``, ``value``
                must be a sequence of values. Optional.
            filter (class:`~google.cloud.firestore_v1.base_query.BaseFilter`): an instance of a Filter.
                Either a FieldFilter or a CompositeFilter.
        Returns:
            :class:`~google.cloud.firestore_v1.query.Query`:
            A filtered query.
        Raises:
            ValueError, if both the positional arguments (field_path, op_string, value)
                and the filter keyword argument are passed at the same time.
        """

Should be fairly simple to extend the mock where call to have same behaviour. Any objections?

[thromer]

I submitted a pull request to support the filter keyword for FieldFilter objects here: #82