Add flash helper constructor to Uri (#7105)

* Update Uri.h

* Add a simple test for the new constructor

Convert a c-str to a FPSTR in the example to have a section
of code in our CI that will catch any future breaks of this specific
kind.

Co-authored-by: Earle F. Philhower, III <earlephilhower@yahoo.com>

libraries/ESP8266WebServer/examples/PathArgServer/PathArgServer.ino

@@ -37,7 +37,7 @@ void setup(void) {
     Serial.println("MDNS responder started");
   }
 
-  server.on("/", []() {
+  server.on(F("/"), []() {
     server.send(200, "text/plain", "hello from esp8266!");
   });