Validate incoming JSON data from pubsub.

[robertwb]

As well as good practice, not doing so may result in much more obscure errors (e.g. during encoding) downstream.

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[codecov]

Codecov Report

Merging #29080 (b97c7de) into master (e7a6405) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master   #29080      +/-   ##
==========================================
- Coverage   38.39%   38.37%   -0.02%     
==========================================
  Files         686      686              
  Lines      101651   101675      +24     
==========================================
- Hits        39025    39021       -4     
- Misses      61042    61075      +33     
+ Partials     1584     1579       -5     

Flag	Coverage Δ
python	29.99% <0.00%> (-0.02%)	⬇️

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Files	Coverage Δ
sdks/python/setup.py	0.00% <ø> (ø)
sdks/python/apache_beam/yaml/yaml_io.py	0.00% <0.00%> (ø)
sdks/python/apache_beam/yaml/json_utils.py	0.00% <0.00%> (ø)

... and 4 files with indirect coverage changes

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[robertwb]

Thanks. I'm re-generating the requirements files to resolve the merge conflicts.