Handle extra columns present on Spanner

[Deep1998]

Description

This PR supports Spanner having extra columns not present at source. Earlier, an additional column was a TODO, so apart from the shard id, extra cols would cause the transformation to error out. Additional columns on Spanner can be populated via

1. Custom transformations
2. Autogeneration like generated columns, default columns etc. (These are now omitted from the mutation)
3. Synthetic PK (These are populated with a UUID like the live template)
4. ShardId (This was already handled previously)

Testing

Unit Tests Added

Pending

• Manual migration using extra columns on Spanner.

[codecov]

Codecov Report

Attention: Patch coverage is 82.22222% with 8 lines in your changes missing coverage. Please review.

Project coverage is 52.86%. Comparing base (15609c3) to head (02818d3).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
.../spanner/migrations/schema/SessionBasedMapper.java	78.57%	6 Missing ⚠️
...er/migrations/avro/GenericRecordTypeConvertor.java	86.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1995      +/-   ##
============================================
+ Coverage     45.26%   52.86%   +7.59%     
+ Complexity     3632     1345    -2287     
============================================
  Files           836      372     -464     
  Lines         49872    20616   -29256     
  Branches       5253     2088    -3165     
============================================
- Hits          22577    10898   -11679     
+ Misses        25621     9041   -16580     
+ Partials       1674      677     -997     

Components	Coverage Δ
spanner-templates	67.86% <82.22%> (+1.23%)	⬆️
spanner-import-export	∅ <ø> (∅)
spanner-live-forward-migration	75.93% <82.22%> (+0.06%)	⬆️
spanner-live-reverse-replication	76.69% <82.22%> (+0.05%)	⬆️
spanner-bulk-migration	86.38% <82.22%> (-0.04%)	⬇️

Files with missing lines	Coverage Δ
...t/v2/spanner/migrations/schema/IdentityMapper.java	100.00% <100.00%> (ø)
...er/migrations/avro/GenericRecordTypeConvertor.java	87.24% <86.66%> (-0.10%)	⬇️
.../spanner/migrations/schema/SessionBasedMapper.java	95.62% <78.57%> (-4.38%)	⬇️

... and 481 files with indirect coverage changes

[bharadwaj-aditya]

Can you also check if this works for the tokenList use case ?

[Deep1998]

TOKENLIST is a new datatype in Spanner that is not handled here. That will be taken up in a follow up PR.

[bharadwaj-aditya]

LGTM