sources: add non-free-firmware component
Some Debian packages moves to non-free-firmware instead of non-free:
- https://lists.debian.org/debian-devel/2022/12/msg00176.html
- https://lists.debian.org/debian-project/2023/01/msg00018.html
We need to track this component to ensure some of our packages are up to date (e.g. raspi-firmware or firmware-nonfree).