Implement the logic related to the Group entity

[krschan]

Develop API endpoints to allow users to create and manage groups.

Tasks:

1. Create Group Entity and Repository

   • Define Group.java with fields: id, name, members.
   • Implement GroupRepository.java for group management.

2. Create GroupController

   • Define the following API endpoints in GroupController.java:
     • POST /api/groups → Create a new group.
     • PUT /api/groups/{id}/add-member → Add a user to a group.
     • DELETE /api/groups/{id}/remove-member → Remove a user from a group.

3. Implement GroupService

   • Create GroupService.java for handling group logic.
   • Ensure users cannot join the same group multiple times.

4. Test Group API Endpoints

   • Write unit tests for GroupService.java.
   • Create integration tests for GroupController.java.