HSv3 descriptor support in stem [decoding]

Hello,

it would be great if stem could support v3 onion service descriptors: both to parse them and to generate them.

This would be particularly useful as part of onionbalance v3 (#26768 (moved)). The old onionbalance actually generates the descriptors itself in an ad-hoc way, but it would be great if we could have stem make them in the new one.

Damian asked me for an HSFETCH command that will fetch a stable v3 onion desc. I used Donncha's script from here (https://gist.github.com/DonnchaC/13b744a1e30b7d34bc26) like this: python tmp/hsfetch.py vww6ybal4bd7szmgncyruucpgfkqahzddi37ktceo3ah7ngmcopnpyyd.onion and that gives me the riseup onion descriptor (there is no DDG v3 onion right now).

Also, v3 descriptors are more complicated than v2 descriptors because of their double-layer encryption. You can see more info here: https://github.com/torproject/torspec/blob/master/rend-spec-v3.txt#L1046 https://github.com/torproject/tor/blob/0acfd7dcee2a4473eba05a53d6df2d6d4fe2050b/src/feature/hs/hs_descriptor.c#L2639

Damian, let me know how that looks like to you and how we can help. We plan to start working on Onionbalance v3 at mid-to-late August, but if this is something we can have at a reasonaable timeframe we can potentially delay the descriptor parts of it for some time until stem support exists.