Support optimized load balancing via HTTP/3 (udp)

[jfallows]

Describe the desired outcome from the user's perspective As a developer, I want to take advantage of http/3 support in browsers, mobile platforms and server environments.

Acceptance criteria

• http/3 clients can send requests and receive responses to and from zilla
• zilla can send requests and receive responses to and from http/3 servers
• zilla can proxy http/1.1 or http/2 protocols to and from http/3 protocol
• zilla can proxy http/3 protocol to and from http/1.1 or http/2 protocols
• zilla can use the same http binding to support http-specific routing for all 3 protocol versions
• zilla can optionally redirect http/3 clients to a peer zilla instance to balance load

Additional context https://caniuse.com/http3

Tasks

• [ ] binding-udp handles udp datagram channels mapping to streams
  • [ ] support multicast datagram channels
• [ ] binding-quic handles network protocol codec mapping to quic application streams
  • mapped to quic application streams
• [ ] binding-http enhanced to support quic network streams
  • mapped to http application streams
  • defers majority of flow control to quic binding
• advertise h3 alternate protocol to bypass load balancer intermediary, requires direct reachability
  • [ ] via alt-svc h2 frame
  • [ ] via alt-svc http/1.1 response header
• [ ] load balance http binding within same auto-scaling group
  • discovery via load balancer can be simple, e.g. round robin or hash tcp syn packet
  • load shaping requires state shared across group for candidates with available capacity (via metrics?)