Add a new userToOrg onboarding function to backend

[akh1lk]

Branch: backend-new-userToOrg

Task at Hand

1. Implement the userToOrg function in user.ts, and it's respective route.
2. Test the function in Postman.
3. Confirm that your code and pull request follow the Pull Request Checklist.

Acceptance Criteria

• This function must have input validation, and the route must work properly with the correct status codes.