Adapt C Tor code to prop359 cell format

I think the right way to do this will be to adapt some of @dgoulet's branch https://gitlab.torproject.org/dgoulet/tor/-/tree/ticket40791_049_03?ref_type=heads, which includes the transition to using a relay_msg_t type throughout the code.

This is the C-tor equivalent of arti#1944 (closed).