feat(spanner): Add Google Spanner executor

[picone]

Summary

This PR adds a new spanner executor to run SQL statements against Google Cloud Spanner from Venom test suites.

What’s included

• New step type: type: spanner
• Inputs:
  • project, instance, database (required)
  • credentials_file (optional; defaults to Application Default Credentials)
  • commands (list of SQL statements) or file (fallback when commands is empty)
• Execution:
  • Statements run sequentially
  • DML is detected by leading keyword (INSERT/UPDATE/DELETE) and executed in a read-write transaction
  • Other statements are treated as queries
• Output:
  • Per statement result in result.queries[] with:
    • statement_type: query + rows: []
    • statement_type: dml + row_count: <number>

Example

- type: spanner
  project: my-project
  instance: my-instance
  database: my-database
  commands:
    - "SELECT SingerId, FirstName FROM Singers LIMIT 1"
    - "UPDATE Singers SET FirstName = 'Alice' WHERE SingerId = 1"