Alt-Svc advertising middleware.

State: #{value => Value}. Injects Alt-Svc: Value on responses served over H1 and H2 so clients can race up to H3 on the next request. Responses already served over H3 are passed through unchanged.

Wired automatically by livery:start_service/1 when the service config sets alt_svc => advertise and an http3 listener is configured.