[AnomalyDetection] Refactor code and add more docstrings

[shunping]

• Move threshold dofn to transforms.py so all DoFns and auxiliary transforms are in one place.
• Add constants for default normal/outlier/missing labels.
• Add docstrings for some specifiable functions.

depends on #34232 and #34234

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[shunping]

r: @damccorm

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[damccorm]

Generally LGTM, lets wait until #34234 is in/we can rebase against master since this pulls in changes from that PR as well.

[codecov]

Codecov Report

Attention: Patch coverage is 97.61905% with 1 line in your changes missing coverage. Please review.

Project coverage is 56.35%. Comparing base (5d38b8e) to head (8b89095).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
sdks/python/apache_beam/ml/anomaly/transforms.py	96.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #34235      +/-   ##
============================================
+ Coverage     56.32%   56.35%   +0.03%     
  Complexity     3286     3286              
============================================
  Files          1172     1173       +1     
  Lines        178933   179045     +112     
  Branches       3398     3398              
============================================
+ Hits         100778   100895     +117     
+ Misses        74898    74893       -5     
  Partials       3257     3257              

Flag	Coverage Δ
python	81.33% <97.61%> (+0.03%)	⬆️

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[damccorm]

Thanks!