prop224: Rendezvous circuit e2e crypto
In order to establish an end-to-end crypto channel on a rendezvous circuit, we need a new API that setups the
crypt_path_t object on the circuit from the key material computed between the client and service. This API has to be for both service and client side.
Because the service implementation in legacy/trac#20657 (moved) is being done before client, setting parent ID to that ticket.