Basic yaml-defined provider.

[robertwb]

This allows one to define new transforms with YAML that can be imported and used in other pipelines.

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[robertwb]

R: @Polber

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Codecov Report

Attention: Patch coverage is 82.85714% with 6 lines in your changes missing coverage. Please review.

Project coverage is 71.40%. Comparing base (f64aec2) to head (8db31ea). Report is 40 commits behind head on master.

Files	Patch %	Lines
sdks/python/apache_beam/yaml/yaml_provider.py	82.85%	6 Missing :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #31684      +/-   ##
============================================
+ Coverage     71.37%   71.40%   +0.02%     
  Complexity     1474     1474              
============================================
  Files           900      900              
  Lines        114202   114306     +104     
  Branches       1076     1076              
============================================
+ Hits          81515    81616     +101     
- Misses        30659    30662       +3     
  Partials       2028     2028              

Flag	Coverage Δ
python	81.30% <82.85%> (+0.02%)	:arrow_up:

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[robertwb]

Ping @Polber