HKG: split CAN and CANFD CarController update function

[royjr]

This pull request addresses #1899 by refactoring the CarController update functions for HKG vehicles, separating the logic for CAN and CANFD communication protocols. The primary changes include:

• Separated update functions for CAN and CANFD to handle their respective messaging protocols.
• Introduced compute_common_controls to manage shared logic between CAN and CANFD, reducing redundancy.
• Moved process_hud_alert inside the class, as it was not used externally.
• Refactored code structure and ordering for better readability and maintainability.

[sshane]

this seems a bit cleaner: #1990

feel free to keep going if you see anything else