DM-33963: add TaskMetadata.get() and improve Pipeline YAML output

[TallJimbo]

All these minor changes have in common is that they were useful for adding a particular new PipelineTask and pipelines for it in downstream packages.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

Merging #238 (58c9191) into main (1c50ebb) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #238      +/-   ##
==========================================
+ Coverage   72.17%   72.27%   +0.09%     
==========================================
  Files          60       60              
  Lines        6595     6618      +23     
  Branches     1244     1246       +2     
==========================================
+ Hits         4760     4783      +23     
  Misses       1606     1606              
  Partials      229      229              

Impacted Files	Coverage Δ
python/lsst/pipe/base/_task_metadata.py	98.17% <100.00%> (+0.04%)	⬆️
python/lsst/pipe/base/pipeline.py	63.90% <100.00%> (+0.27%)	⬆️
python/lsst/pipe/base/pipelineIR.py	85.89% <100.00%> (+0.17%)	⬆️
tests/test_pipelineIR.py	98.50% <100.00%> (+0.03%)	⬆️
tests/test_taskmetadata.py	97.64% <100.00%> (+0.07%)	⬆️
python/lsst/pipe/base/graph/graph.py	81.23% <0.00%> (ø)

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