Set an auth key in multi_process_shared.py

[damccorm]

Right now, multi_process_shared doesn't set an auth key. When that happens, the manager still creates an auth key for that process based on the current process's authKey - https://github.com/python/cpython/blob/4dc339b4d69195448207e1faecc3e258700daf33/Lib/multiprocessing/managers.py#L503

This causes cross-process authentication to fail and means that multi_process_shared.py doesn't correctly work.

This PR fixes this by using a constant auth key for all managers/requesters/processes.

I was able to get an example running on Dataflow using this approach. #26178 also causes unit tests to fail which will be fixed by this change, so I'm deferring tests to that PR

Fixes #26167

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[codecov]

Codecov Report

Merging #26172 (1a6fc7c) into master (87db1ae) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #26172      +/-   ##
==========================================
- Coverage   71.17%   71.16%   -0.01%     
==========================================
  Files         787      787              
  Lines      103294   103319      +25     
==========================================
+ Hits        73523    73532       +9     
- Misses      28283    28299      +16     
  Partials     1488     1488              

Flag	Coverage Δ
python	79.85% <100.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
...s/python/apache_beam/utils/multi_process_shared.py	97.43% <100.00%> (+0.06%)	⬆️

... and 7 files with indirect coverage changes

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[damccorm]

R: @robertwb

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Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[damccorm]

retest this please

[damccorm]

retest this please

[robertwb]

Thanks for fixing this.