Port UserGroup to Use Eloquent Model

[Hafsa-Naeem]

Issue Description

The current implementation of the UserGroup does not utilize Eloquent. This limitation makes it hard to manage relationships, run complex queries efficiently and perform eager loading, especially when adding new fields to the Submission API (Issue #10480)

Proposed Solution

Refactor UserGroup class to use Eloquent Model and define appropriate Eloquent relationships. This will enable the use of eager loading (StageAssignment::with('userGroup')) to optimize data retrieval, reduce the number of database queries, and ensure accurate computation of the additional fields required for the Submission API.