Open methods to allow custom static file serving logic (#10910)

* Open methods to allow custom static file serving logic

The methods serveStaticResources and serveStaticResourcesInVAADIN have
been changed from private to protected to allow subclasses to change how
static files are served.

server/src/main/java/com/vaadin/server/VaadinServlet.java

      *         otherwise.
      * @throws IOException
      * @throws ServletException
+     * 
+     * @since
      */
-    private boolean serveStaticResources(HttpServletRequest request,
+    protected boolean serveStaticResources(HttpServletRequest request,
             HttpServletResponse response) throws IOException, ServletException {
 
         String pathInfo = request.getPathInfo();
      * @param response
      * @throws IOException
      * @throws ServletException
+     * 
+     * @since
      */
-    private void serveStaticResourcesInVAADIN(String filename,
+    protected void serveStaticResourcesInVAADIN(String filename,
             HttpServletRequest request, HttpServletResponse response)
             throws IOException, ServletException {