Applies the given window function to each window. The window function is called for each
evaluation of the window for each key individually. The output of the window function is
interpreted as a regular non-windowed stream.
Note that this function requires that all data in the windows is buffered until the window
is evaluated, as the function provides no means of incremental aggregation.
process_window_function – The window function.
output_type – Type information for the result type of the window function.
The data stream that is the result of applying the window function to the window.