feat: Integrate Forum V2 into edx-platform

[regisb]

This is a backport of the following PR #35671

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 discussions.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.

Description

Describe what this pull request changes, and why. Include implications for people using this change.
Design decisions and their rationales should be documented in the repo (docstring / ADR), per
OEP-19, and can be
linked here.

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Thanks for the pull request, @regisb!

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