Add fallback to GitHub AUR mirror when aur.archlinux.org is down

Description:

Currently, paru relies only on aur.archlinux.org. When aur.archlinux.org is down (e.g. high load or DDoS attack), paru cannot search or fetch packages.

The Arch Linux team provides an GitHub mirror of AUR packages:

git clone --branch <package_name> --single-branch https://github.com/archlinux/aur.git <package_name>

Question / Feature idea: Would it be possible for paru to automatically fall back to the GitHub AUR mirror when aur.archlinux.org is unreachable?

Benefit: This could improve the reliability of paru during AUR downtime.