feat(spanner): add DML statement support in migration files

[jferrl]

Summary

• Add support for DML statements (INSERT, UPDATE, DELETE) in Spanner migration files
• Implement automatic statement type detection using spansql parser
• Deprecate CleanStatements config flag (now always parses statements)

Changes

New Features

• DDL statements (CREATE, ALTER, DROP): Executed via UpdateDatabaseDdl API
• DML INSERT statements: Executed within a read-write transaction
• Partitioned DML (UPDATE, DELETE): Executed via PartitionedUpdate API for large-scale modifications
• SQL comments (-- and /* */) are automatically stripped during parsing

Breaking Changes

• Migration files must contain only one statement type (cannot mix DDL with DML, or INSERT with UPDATE/DELETE)

Deprecations

• CleanStatements config field is deprecated and ignored
• x-clean-statements URL parameter is deprecated and ignored

Test plan

• Unit tests for parseStatements() function (18 test cases)
• Unit tests for inspectDMLKind() function (6 test cases)
• Integration tests with example migration files
• All existing tests pass

References

Implementation based on wrench migration handling.

Closes #135
Closes #775
Closes #918
Closes #1194

[jferrl]

@dhui please, could you take a look ? Thanks! 😄

[coveralls]

coverage: 54.627% (+0.2%) from 54.432%
when pulling 3ff5793 on jferrl:jferrl-support-dml
into 89e308c on golang-migrate:master.

[jferrl]

After updating to a recent version of spanner client, there a re several open PRs that are no longer needed and issues that can be closed.