Add WindowedValueParam option to DoFns.

[robertwb]

This is useful when one wants to capture and preserve all the windowing information for later use (and is both more convenient and less error-prone than gathering the timestamp, windows, and pane info separately).

New values with the same windowing value can be constructed via WindowedValue.with_value(...) and emitted asynchronously (e.g. after batching or in finish_bundle).

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[robertwb]

R: @ganesh4991

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 58.81%. Comparing base (e62daf7) to head (9771a42). Report is 49 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #32305   +/-   ##
=========================================
  Coverage     58.81%   58.81%           
  Complexity     3023     3023           
=========================================
  Files          1123     1123           
  Lines        173452   173459    +7     
  Branches       3301     3301           
=========================================
+ Hits         102021   102026    +5     
- Misses        68099    68101    +2     
  Partials       3332     3332           

Flag	Coverage Δ
python	81.62% <100.00%> (-0.01%)	:arrow_down:

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