Draft proposal document for Arti integration

@dan @boyska and @morgan met a couple weeks ago to chat about future arti integration plans. One takeaway task was for @morgan to consolidate the 3 or 4 arti needs document into a coherent proposal for the shape of a usable dev API layer which would work for each of our usecases.

Currently the dev requirements are scattered across multiple places:

• arti RPC design meta: tpo/core/arti#1423
• Tor Browser: tpo/core/arti#1303
• Tails: https://forum.torproject.org/t/defining-an-interface-to-arti/6432/7
• Cwtch: tpo/core/arti!2388 (comment 3075837)

cc @micah