add flat list option to rpc.methods.collections()

    recursive: bool = False,

    limit_from: int = 0,

    limit_to: int = 2000,

    flat_list: bool = False,

) -> List[Dict]:

    """

    Return the user's collections under the parent collection

    specified by 'parent_id'. If 'parent_id' is null, will return

    collections available at root. If 'recursive' is true, will

    return all the descendants recursively in the 'children' key.

    If recursive is false, 'children' is null.

    If recursive is false, 'children' is null. If flat_list is True,

    collections are returned as a flat list and parent is not used.

    Example output:

    if limit_to < limit_from:

        return []

    data = []

    if flat_list:

        query_set: Union[Iterator[Collection], QuerySet] = Collection.objects.filter(

            owner=user, deleted_at__isnull=True

        )[limit_from:limit_to]

    else:

        query_set: Union[Iterator[Collection], QuerySet] = Collection.objects.filter(

            owner=user, parent=parent, deleted_at__isnull=True

        )[limit_from:limit_to]