Add Seekable Compressor (Random Access)

[lechiefe]

Hi, I was wondering how hard it would be to add the seekable functionality that was added to /contrib (https://github.com/sean-purcell/zstd/tree/splittable/contrib/seekable_format) ?

I am currently using a Gzip extension called dictzip which allows for random access, but trying to switch over to zstd.

Thanks!