erofs-utils: add preliminary zstd support [x]

erofs-utils: add preliminary zstd support [x]

This patch just adds a preliminary Zstandard support to erofs-utils
since currently Zstandard doesn't support fixed-sized output compression
officially.  Mkfs could take more time to finish but it works at least.

The built-in zstd compressor for erofs-utils is slowly WIP, therefore
apparently it will take more efforts.

[ TODO: Later I tend to add another way to generate fixed-sized input
        pclusters temporarily for relatively large pcluster sizes as
        an option since it will have minor impacts to the results. ]

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240515051641.3929058-1-hsiangkao@linux.alibaba.com