Request adding support for zstd compressed squashfs files.

[blickers]

Zstd is well known for its high compression ratio, much faster decompression speed and lower CPU usage. Squashfs has already supported zstd compression for several years and now the newly released Linux kernel 5.19 can boot from zstd compressed squashfs firmwares. But GRUB2's squash4 module still does not support reading zstd compressed squashfs files, making it not possible to load kernels inside zstd-compressed squashfs files and boot the squashfs compressed live system.

So adding zstd to GRUB2's squash4 module is really important and necessary. It should not be difficult because GRUB2's Btrfs module already supports zstd decompression. I don't know C/C++. If I do, I would like to do this job and contribute my code.