[#32115] Fix timer support, support timer clears.

[lostluck]

• Prism wasn't decoding all timers returned in the logical timer stream, dropping later set timers.
  • Moved timer decoding to be iterator compatible for the future, instead of trying to reprocess inplace, or returning the remaining byte buffer.
• From there supporting clearing timers was simple, by additionally returning the window set, and deleting existing entries to the pending timer list.
• Needed to re-write + override the timer clearing test, since for some reason the test DoFn would not execute when there were multiple timer parameters to the DoFn.

Allows a few of the Python timer based tests to pass.

TODO for a different test: Support interval and custom window functions for timers.

Fixes #32115

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[lostluck]

R: @damondouglas

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[codecov]

Codecov Report

Attention: Patch coverage is 70.49180% with 18 lines in your changes missing coverage. Please review.

Project coverage is 58.68%. Comparing base (adc3b2b) to head (33946a3).
Report is 6 commits behind head on master.

Files	Patch %	Lines
...o/pkg/beam/runners/prism/internal/engine/timers.go	71.42%	12 Missing and 2 partials ⚠️
...am/runners/prism/internal/engine/elementmanager.go	66.66%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #32119   +/-   ##
=========================================
  Coverage     58.68%   58.68%           
  Complexity     3025     3025           
=========================================
  Files          1122     1122           
  Lines        172906   172923   +17     
  Branches       3285     3285           
=========================================
+ Hits         101463   101476   +13     
- Misses        68131    68134    +3     
- Partials       3312     3313    +1     

Flag	Coverage Δ
go	34.63% <70.49%> (+<0.01%)	⬆️

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[damondouglas]

I admit I didn't understand what a lot of this code was doing. From a reviewer perspective, I'm approving to unblock given there is at least a testing coverage somewhere.