[YAML] Add PubSub reading and writing transforms.

[robertwb]

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[codecov]

Codecov Report

Merging #28595 (f7039fa) into master (c5e6c79) will decrease coverage by 0.01%.
Report is 34 commits behind head on master.
The diff coverage is 87.62%.

@@            Coverage Diff             @@
##           master   #28595      +/-   ##
==========================================
- Coverage   72.24%   72.24%   -0.01%     
==========================================
  Files         684      684              
  Lines      101071   101232     +161     
==========================================
+ Hits        73022    73136     +114     
- Misses      26470    26517      +47     
  Partials     1579     1579              

Flag	Coverage Δ
python	82.78% <87.62%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
sdks/python/apache_beam/testing/util.py	96.79% <100.00%> (+0.08%)	⬆️
sdks/python/apache_beam/typehints/schemas.py	88.91% <100.00%> (-0.26%)	⬇️
sdks/python/apache_beam/yaml/yaml_transform.py	90.11% <100.00%> (+0.07%)	⬆️
sdks/python/apache_beam/transforms/core.py	92.70% <84.61%> (+1.16%)	⬆️
sdks/python/apache_beam/yaml/yaml_io.py	69.16% <86.84%> (+29.16%)	⬆️

... and 15 files with indirect coverage changes

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[github-actions]

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[liferoad]

similarly, id_label?

[robertwb]

Done.

[liferoad]

LGTM. Please fix the failed tests before merging.