zstd compression level.

[MariasStory]

Please give an easy way to control the zstd compression level. It is tunable with compression levels ranging from negative 7 (fastest) to 22 (slowest in compression speed, but best compression ratio). Compression speed can vary by a factor of 20 or more between the fastest and slowest levels, while decompression is uniformly fast, varying by less than 20% between the fastest and slowest levels. Ref: https://en.wikipedia.org/wiki/Zstd