Introduce MASQUE, part 2: client code This CL introduces the client 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: 285443244 Change-Id: I76c66a4d89b8b70aada4f15f03ac5ec139ada22f