Refactor BulkTester to support database-specific operations

[AshAnand34]

Issue resolved: #13

• Updated startDatabases method to accept a database parameter for targeted container startup.
• Introduced killDatabase method to stop individual database containers.
• Modified runDataGeneration and runQueryTest methods to handle database-specific data generation and query testing.
• Enhanced the main testing loop to iterate over configured databases, ensuring proper startup, data generation, query testing, and cleanup for each database.
• Updated start-databases utility to accept a --databases option for selective container startup.

This improves the flexibility and usability of the bulk testing process by allowing users to specify which databases to start and test.