reverseproxy: Enable response interception (#1447, #2920)

It's a raw, low-level implementation for now, but it's very flexible.
More sugar-coating can be added after error handling is more developed.

modules/caddyhttp/subroute.go

@@ -80,6 +80,36 @@ func (sr *Subroute) ServeHTTP(w http.ResponseWriter, r *http.Request, next Handl
 	return err
 }
 
+// ResponseHandler pairs a response matcher with a route list.
+// It is useful for executing handler routes based on the
+// properties of an HTTP response that has not been written
+// out to the client yet.
+//
+// To use this type, provision it at module load time, then
+// when ready to use, match the response against its matcher;
+// if it matches (or doesn't have a matcher), invoke the routes
+// by calling `rh.Routes.Compile(next).ServeHTTP(rw, req)` (or
+// similar).
+type ResponseHandler struct {
+	// The response matcher for this handler. If empty/nil,
+	// it always matches.
+	Match *ResponseMatcher `json:"match,omitempty"`
+
+	// The list of HTTP routes to execute.
+	Routes RouteList `json:"routes,omitempty"`
+}
+
+// Provision sets up the routse in rh.
+func (rh *ResponseHandler) Provision(ctx caddy.Context) error {
+	if rh.Routes != nil {
+		err := rh.Routes.Provision(ctx)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 // Interface guards
 var (
 	_ caddy.Provisioner = (*Subroute)(nil)