Consider using a binary format for the index

Given that we're doing a new index format, it would be good to consider some format other than json. protobuf, cbor, to name a couple.

du -bsh https://f-droid.org/repo/$file
6.1M    index-v1.jar
26M     index-v1.json

That's huge. It's not quick to download, nor quick to process. Perhaps a binary format would be faster to process.

Might help with #2345 too (if not with file-sizes, then maybe with processing time)