Refactoring the guest to user convertion

[gustaavik]

This pull request includes several changes to the AuthenticationController, UserManagementController, AuthenticationService, and UserManagementService to improve user management and authentication processes. The most notable changes are the addition of a new endpoint for registering guests, the refactoring of guest conversion logic, and the removal of redundant code.

Authentication and Authorization Enhancements:

• Added @PreAuthorize annotation to AuthenticationController to ensure that only users with the appropriate roles can access the new registerGuest endpoint. [1] [2]
• Introduced a new registerGuest method in AuthenticationController to convert guest users to registered users.

Code Refactoring:

• Moved the registerGuest logic from UserManagementController to AuthenticationController and removed the corresponding method from UserManagementController.
• Refactored AuthenticationService to include a new method convertGuestToUser for handling guest conversion logic.

Code Cleanup:

• Removed unused imports and redundant code from UserManagementService. [1] [2] [3] [4]
• Removed debug System.out.println statements from OrderService.