Experimental IETF-standard HTTP/3 support (known issue exists) (#2727)

* Begin WIP integration of HTTP/3 support

* http3: Set actual Handler, make fakeClosePacketConn type for UDP sockets

Also use latest quic-go for ALPN fix

* Manually keep track of and close HTTP/3 listeners

* Update quic-go after working through some http3 bugs

* Fix go mod

* Make http3 optional for now

modules/caddyhttp/server.go

@@ -43,6 +43,9 @@ type Server struct {
 	MaxRehandles      *int                        `json:"max_rehandles,omitempty"`
 	StrictSNIHost     bool                        `json:"strict_sni_host,omitempty"`
 
+	// This field is not subject to compatibility promises
+	ExperimentalHTTP3 bool `json:"experimental_http3,omitempty"`
+
 	tlsApp *caddytls.TLS
 }