Draft: RFC: Add integration of efi-shell-x64 in GRUB menu

This is implemented by adding:

• a postinst script to copy the executable to the ESP and run update-grub.
• a postrm script to remove the executable from the ESP and run update-grub.
• a grub-mkconfig helper script to add a conditional menu entry shown only if the EFI architecture matches.

This MR is a PoC limited to x64 and may be extended to other platforms if it receives positive feedback from maintainers.

See also https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1036948