Windows this data stream to a WindowedStream, which evaluates windows over a key
grouped stream. Elements are put into windows by a WindowAssigner. The grouping of
elements is done both by key and by window.
A Trigger can be defined to specify when windows are evaluated. However, WindowAssigners
have a default Trigger that is used if a Trigger is not specified.
window_assigner – The WindowAssigner that assigns elements to windows.