Add limit and offset support to `get_entities_by_kind`

[joente]

When no GcdModel is used, the function get_entities_by_kind should support an offset, limit and thus cursor to query entities in batches.

Since this requires a cursor, the result of the function should depend on the limit argument. When None, the "old" behavior should be honored for backwards compatibility. When a limit is set, the function should return a (list, cursor) tuple.