Refactor template to avoid using session file for type info

[Deep1998]

This PR refactors the reverse template to avoid using session file for anything other than table/column name-mappings. It instead relies of the Ddl and SourceDdl object for schema specific info. This refactoring is necessary to introduce the schemamapper and henceforth, overrides.

[codecov]

Codecov Report

Attention: Patch coverage is 59.14634% with 67 lines in your changes missing coverage. Please review.

Project coverage is 49.71%. Comparing base (c0fb979) to head (cb13298).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
...cloud/teleport/v2/templates/SpannerToSourceDb.java	0.00%	35 Missing ⚠️
...rt/v2/templates/dbutils/dml/MySQLDMLGenerator.java	72.13%	9 Missing and 8 partials ⚠️
...2/templates/dbutils/dml/CassandraDMLGenerator.java	80.00%	4 Missing and 5 partials ⚠️
...eport/v2/templates/transforms/AssignShardIdFn.java	66.66%	3 Missing and 1 partial ⚠️
...v2/templates/dbutils/dml/CassandraTypeHandler.java	85.71%	0 Missing and 1 partial ⚠️
...v2/templates/transforms/SourceWriterTransform.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2529      +/-   ##
============================================
- Coverage     49.73%   49.71%   -0.03%     
- Complexity     4822     5179     +357     
============================================
  Files           948      948              
  Lines         57938    58038     +100     
  Branches       6259     6275      +16     
============================================
+ Hits          28818    28853      +35     
- Misses        27070    27123      +53     
- Partials       2050     2062      +12     

Components	Coverage Δ
spanner-templates	69.94% <59.14%> (-0.15%)	⬇️
spanner-import-export	68.61% <ø> (+0.13%)	⬆️
spanner-live-forward-migration	79.33% <ø> (+0.10%)	⬆️
spanner-live-reverse-replication	77.02% <59.14%> (-0.78%)	⬇️
spanner-bulk-migration	88.05% <ø> (+0.06%)	⬆️

Files with missing lines	Coverage Δ
...plates/dbutils/processor/InputRecordProcessor.java	86.44% <100.00%> (-1.28%)	⬇️
...leport/v2/templates/transforms/SourceWriterFn.java	85.34% <100.00%> (+0.12%)	⬆️
...v2/templates/dbutils/dml/CassandraTypeHandler.java	84.73% <85.71%> (ø)
...v2/templates/transforms/SourceWriterTransform.java	0.00% <0.00%> (ø)
...eport/v2/templates/transforms/AssignShardIdFn.java	79.59% <66.66%> (+1.86%)	⬆️
...2/templates/dbutils/dml/CassandraDMLGenerator.java	84.96% <80.00%> (-5.10%)	⬇️
...rt/v2/templates/dbutils/dml/MySQLDMLGenerator.java	88.98% <72.13%> (-8.80%)	⬇️
...cloud/teleport/v2/templates/SpannerToSourceDb.java	0.00% <0.00%> (ø)

... and 11 files with indirect coverage changes

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[bharadwaj-aditya]

This changes are very spread out and hard to quantify or review because of the fan out. It would be cleaner to introduce an interface and swap between session file and alternates. It will keep the implementation and testing clean

[bharadwaj-aditya]

Approving as an intermediate PR before the interface changes are merged to simplify this code.