[YAML] Add optional output type parameter to mappings.

[robertwb]

This can be useful when inference is insufficient for Python/Javascript.

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[robertwb]

R: @Polber

[github-actions]

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

[codecov]

Codecov Report

Merging #29077 (ab245e9) into master (40e5c38) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

❗ Current head ab245e9 differs from pull request most recent head 46a3318. Consider uploading reports for the commit 46a3318 to get more accurate results

@@            Coverage Diff             @@
##           master   #29077      +/-   ##
==========================================
- Coverage   38.38%   38.35%   -0.03%     
==========================================
  Files         686      686              
  Lines      101665   101735      +70     
==========================================
+ Hits        39020    39021       +1     
- Misses      61068    61135      +67     
- Partials     1577     1579       +2     

Flag	Coverage Δ
python	29.96% <0.00%> (-0.04%)	⬇️

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

Files	Coverage Δ
sdks/python/apache_beam/transforms/core.py	41.41% <0.00%> (-0.14%)	⬇️
sdks/python/apache_beam/yaml/yaml_mapping.py	0.00% <0.00%> (ø)

... and 6 files with indirect coverage changes

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