feat: Implement 'rebuild' command and refactor core components

[elchinoo]

This commit introduces several major changes:

• New 'rebuild' command: A 'rebuild' boolean flag can now be passed in the test run configuration. When true, the target test database is dropped and recreated before the test begins. This is implemented in the API layer and supported by both the 'bulk-load' and 'tpcc-scalability' plugins.

• Core Component Refactoring:

  • Scheduler: The scheduler is now enabled by default and is the sole component responsible for creating test runs. It correctly resolves plugin and test type IDs from the database, ensuring foreign key constraints are met.
  • Plugin Manager: The plugin manager now automatically registers a plugin's declared test types upon loading. It also ensures the in-memory plugin metadata is updated with the database-assigned ID.
  • Storage: The storage layer was updated to handle the new schema.

• Database Schema:

  • The 'plugin_ver' column was removed from the 'test_run' table, as this information is now correctly linked via the 'plugin' table.
  • A new migration was added to handle the schema change, including dropping and recreating dependent views.

• Build System:

  • Removed nested Go modules from the plugin directories to resolve dependency conflicts.
  • Updated plugin Makefiles to build them as part of the main project module.