[YAML] Avro format for PubSub.

[robertwb]

---

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[codecov]

Codecov Report

Merging #28899 (41114a4) into master (ad5c2f5) will decrease coverage by 0.07%.
Report is 19 commits behind head on master.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #28899      +/-   ##
==========================================
- Coverage   72.24%   72.17%   -0.07%     
==========================================
  Files         685      686       +1     
  Lines      101490   101570      +80     
==========================================
- Hits        73324    73312      -12     
- Misses      26587    26681      +94     
+ Partials     1579     1577       -2     

Flag	Coverage Δ
python	82.62% <100.00%> (-0.13%)	⬇️

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Files	Coverage Δ
sdks/python/apache_beam/yaml/yaml_io.py	74.30% <100.00%> (+3.21%)	⬆️

... and 12 files with indirect coverage changes

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R: @tvalentyn added as fallback since no labels match configuration

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[tvalentyn]

what fails with default pickler?

[robertwb]

Functions instantiated with eval often have trouble with the default pickler (and dill). These come up from PythonCallableWithSource.