Support Zstd seekable format

[lechiefe]

Hi there,
I was wondering if there are any plans to support the additional functionality that the original zstd library has in /contrib (https://github.com/facebook/zstd/tree/dev/contrib/seekable_format).

It allows for random read access to files without the need to decompress the entire file. I am not familiar with C nor with the zstd codebase, but if someone would be willing to give me a bit of guidance I am happy to try and implement it myself.

Thanks