➕ Support Multiple Challenge Versions in Same Repository via Branches

**Description:**

Currently, EvalAI allows only one challenge configuration per repository. To improve challenge management and versioning, we propose supporting **multiple challenge versions hosted in the same repository** using **different Git branches**.

This will help challenge organizers maintain separate versions of the same challenge (e.g., 2023, 2024, and 2025 editions) cleanly without duplicating repositories.

### 📌 Feature Proposal

* Extend support for loading challenge configurations from a **specified branch** instead of just the `challenge`.
* UI/API updates to allow:
* Selecting the branch name during challenge creation.
* Storing and referencing the selected branch in the database.
* Ensure compatibility with existing workflows:

* If no branch is specified, default to `challenge` for challenge creation.

### 🔗 Reference

Related PR: [[Cloud-CV/EvalAI-Starters#78](https://github.com/Cloud-CV/EvalAI-Starters/pull/78)](https://github.com/Cloud-CV/EvalAI-Starters/pull/78) – *Add support for multiple challenge versions via different branches*.