Implement Authentication for Admin Users

[Tumelo2748]

Description:

This PR adds authentication middleware for securing our applications using JWT.

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Key Changes:

1. Admin Controller:

   • Added registerAdmin and loginAdmin controllers to handle authentication logic.

2. Middleware:

   • Added authenticateToken and isAdmin middleware to protect admin-only routes.

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How Has This Been Tested?

• Locally tested:
  • Admin registration and login using JWT tokens.
  • Protecting admin-only routes with authenticateToken and isAdmin middleware.

Related Issue:

• Closes #10

Checklist:

• [x] Created admin registration and login logic to controllers.
• [x] Created middleware to protect admin routes.
• [x] Locally tested and verified.