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 #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 #15516 (moved) as the parameters, initially, come from that ticket work.