hs-v3: Write a proposal for an ESTABLISH_INTRO cell extension containing DoS defense parameters

This ticket is for writing the proposal that will allow an onion service to put DoS defense parameters (at the intro point, like legacy/trac#15516 (moved)) in the ESTABLISH_INTRO cell so the intro point can use those.

It is more than possible that we probably want more than just DoS parameters in that cell.

This is related to legacy/trac#15516 (moved) as the parameters, initially, come from that ticket work.