Refactor modify_sources_list

[Danilop95]

Enhance the script by refactoring the modify_sources_list function and adding several new checks to improve flexibility and user experience.

Objectives:

• Refactor modify_sources_list Function: Update the function to add only missing repositories without overwriting existing entries. This will ensure that the script maintains current configurations and avoids duplicating entries.

• Implement GPU Usage Check for Proxmox Instances with UI: Add logic to detect if the Proxmox instance has a graphical user interface (UI). If a UI is detected, the script should avoid applying changes, such as blacklisting drivers, that could interfere with GPUs used by the desktop environment.

• Check for Enterprise Repository Status: Include a check to determine if enterprise repositories are enabled. This is crucial to ensure that the script respects user subscriptions and does not disable enterprise repositories for those who have purchased them.

• Test and Validate Changes: Perform thorough testing to ensure that the refactored function and new checks work correctly across different Proxmox setups, including both UI and non-UI environments and with or without enterprise repositories.