feat(admin): implement CRUD endpoints for roles and permissions

[indrasuthar07]

Implemented a complete set of API endpoints for managing Roles and Permissions, restricted to Admin users.
This includes:
1.Roles CRUD (/api/roles)
2.Permissions CRUD (/api/permissions)
3.Assigning permissions to roles (/api/roles/:roleId/permissions)

All routes are protected by auth and RBAC middleware, accessible only to users with the Admin role.
Testing Done:
-Tested all endpoints locally using Postman with a valid Admin JWT token.
-Verified responses in Postman: creation of Role, creation of Permission, assigning Permission to Role, and fetching all roles.
Verified database updates in MongoDB Compass:
-roles collection contains the role(s) with assigned permissions.
-permissions collection contains all permissions created.

Screenshots:

Issues

closes #15

Checklist

• I have read the Contributing Guidelines.

[Somilg11]

Thank you for raising a PR. Due to the previous branch merge, there may be some conflicts; do check them out and resolve them. Thanks again for your contribution.
Do git pull -> resolve the conflicts -> commit the changes in the same

[indrasuthar07]

Done sir

[nsh07]

@Somilg11 there is a mismatch in the difficulty level of this PR (medium) and the difficulty level of the corresponding issue (hard), so @indrasuthar07 will only receive points according to the PR's difficulty (medium). Is this intentional? If not, do let me know and I will update the score accordingly.

[Somilg11]

Do update the score, according to type hard