Label Python external SchemaTransform with its URN

[ahmedabu98]

Doing this so that we can identify the transform URN when looking at a pipeline DAG.

[codecov]

Codecov Report

Merging #28540 (1a1a6e2) into master (1225dee) will decrease coverage by 0.01%.
Report is 137 commits behind head on master.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master   #28540      +/-   ##
==========================================
- Coverage   72.33%   72.32%   -0.01%     
==========================================
  Files         682      683       +1     
  Lines      100541   100821     +280     
==========================================
+ Hits        72723    72920     +197     
- Misses      26243    26326      +83     
  Partials     1575     1575              

Flag	Coverage Δ
python	82.81% <50.00%> (-0.06%)	⬇️

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Files Changed	Coverage Δ
sdks/python/apache_beam/transforms/external.py	74.26% <50.00%> (-0.10%)	⬇️

... and 27 files with indirect coverage changes

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

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[ahmedabu98]

stop reviewer notifications

[github-actions]

Stopping reviewer notifications for this pull request: requested by reviewer

[chamikaramj]

LGTM. Thanks.