This commit introduces the new Forum V2 application, allowing users to choose between the legacy Forum V1 and the new Forum V2 at the course level.

Key Changes:
- Added waffle flag `forum_v2.enable_forum_v2` to enable Forum V2 for selected courses, allowing coexistence with Forum V1.
- Default data storage for Forum V2 is set to MongoDB, with an option to switch to MySQL using the waffle flag `forum_v2.enable_mysql_backend`.
- Introduced management command `forum_migrate_course_from_mongodb_to_mysql` for per-course data migration from MongoDB to MySQL.

Note: This PR does not include all unit tests for the Forum V2 native API due to ongoing migration efforts. Further updates will follow to ensure full test coverage before final release.

Co-authored-by: [Muhammad Faraz Maqsood] <faraz.maqsood@arbisoft.com>
Co-authored-by: [Ali Salman] <ali.salman@arbisoft.com>