Introduce MASQUE, part 3: server code This CL introduces the server code for MASQUE as defined by <https://tools.ietf.org/html/draft-schinazi-masque>. Most of the work here is plumbing in order to override the right methods of the QUIC codebase. The meat of the MASQUE protocol work is in the parent cl/278956073. gfe-relnote: n/a, adds unused code PiperOrigin-RevId: 286093484 Change-Id: Ia05871196f7ae88ece9703f24bc4032f93220889