caddyhttp: Make use of http.ResponseController (#5654)

* caddyhttp: Make use of http.ResponseController

Also syncs the reverseproxy implementation with stdlib's which now uses ResponseController as well https://github.com/golang/go/commit/2449bbb5e614954ce9e99c8a481ea2ee73d72d61

* Enable full-duplex for HTTP/1.1

* Appease linter

* Add warning for builds with Go 1.20, so it's less surprising to users

* Improved godoc for EnableFullDuplex, copied text from stdlib

* Only wrap in encode if not already wrapped

modules/caddyhttp/push/handler.go

@@ -251,5 +251,6 @@ const pushedLink = "http.handlers.push.pushed_link"
 var (
 	_ caddy.Provisioner           = (*Handler)(nil)
 	_ caddyhttp.MiddlewareHandler = (*Handler)(nil)
-	_ caddyhttp.HTTPInterfaces    = (*linkPusher)(nil)
+	_ http.ResponseWriter         = (*linkPusher)(nil)
+	_ http.Pusher                 = (*linkPusher)(nil)
 )