feat(Spanner): Live Migration: Remove generated columns from change events during transformation and add tests for this behavior

[pratickchokhani]

If a primary is Generated Column within Spanner and the change stream event is DELETE, then DML will be used
Else Mutation is used.

For DML, we include all the non-generated columns in the where clause. This is so because we do not have the list of dependent columns.

If column is Generated Column in Spanner, then the column is omitted from both DML or Mutations.

[codecov]

Codecov Report

❌ Patch coverage is 79.74684% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.28%. Comparing base (d116909) to head (385cdaa).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
...ort/v2/templates/SpannerTransactionWriterDoFn.java	11.11%	7 Missing and 1 partial ⚠️
...rt/v2/templates/datastream/ChangeEventContext.java	91.83%	0 Missing and 4 partials ⚠️
...igrations/convertors/ChangeEventTypeConvertor.java	71.42%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3255      +/-   ##
============================================
+ Coverage     51.05%   55.28%   +4.22%     
+ Complexity     5212     2291    -2921     
============================================
  Files           994      492     -502     
  Lines         60723    28947   -31776     
  Branches       6657     3076    -3581     
============================================
- Hits          31000    16002   -14998     
+ Misses        27543    11966   -15577     
+ Partials       2180      979    -1201     

Components	Coverage Δ
spanner-templates	71.96% <79.74%> (+1.11%)	⬆️
spanner-import-export	∅ <ø> (∅)
spanner-live-forward-migration	79.85% <79.74%> (+<0.01%)	⬆️
spanner-live-reverse-replication	77.33% <80.00%> (-0.04%)	⬇️
spanner-bulk-migration	87.99% <80.00%> (-0.01%)	⬇️
gcs-spanner-dv	85.73% <80.00%> (-0.03%)	⬇️

Files with missing lines	Coverage Δ
...eport/v2/templates/spanner/ShadowTableCreator.java	89.36% <100.00%> (+0.23%)	⬆️
...ations/convertors/ChangeEventSpannerConvertor.java	85.24% <100.00%> (+2.08%)	⬆️
...rt/v2/templates/datastream/ChangeEventContext.java	92.47% <91.83%> (+1.16%)	⬆️
...igrations/convertors/ChangeEventTypeConvertor.java	88.05% <71.42%> (-1.95%)	⬇️
...ort/v2/templates/SpannerTransactionWriterDoFn.java	50.43% <11.11%> (-1.37%)	⬇️

... and 520 files with indirect coverage changes

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[shreyakhajanchi]

Added some comments. Please add details of manual test run in the description for completeness.

[shreyakhajanchi]

LGTM