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.

[PhamelaMhlaba]

Great job on implementing the authentication system for admin users using JWT! I see you have added the registerAdmin and loginAdmin controllers and middleware like authenticateToken and isAdmin to secure the routes.

Since I'm still learning, I have a quick question: How does the authenticateToken middleware handle cases when a token is invalid?

Thanks for taking the time for testing the features locally!

[Tumelo2748]

So Jwt use verify to check if a token is valid or not, It check whether the token is expired or not Another way it checks if a token is valid or not is by checking if the jwt matches with the secret key