zchunk notes

[wolfv]

If we want to deliver zchunk repodata, we need to have additional repodata-metadata.

• zchunk needs header size and header checksum (SHA256 for example) before the download start
• for repodata it would make sense to create a zchunk dict https://github.com/zchunk/zchunk#zchunk-dictionaries
• zchunk extraction is also implemented in libsolv
• we can download zck_get_min_download_size bytes, and then figure out the necessary header metadata