allow last modified to be also set to time_t or current time

to start sntp:
```cpp
void startSNTP(){
  time_t rawtime;
  configTime(3 * 3600, 0, "pool.ntp.org", "time.nist.gov");
  while(time(&rawtime) == 0) delay(10);
}
```

src/WebHandlerImpl.h

@@ -49,6 +49,8 @@ class AsyncStaticWebHandler: public AsyncWebHandler {
     AsyncStaticWebHandler& setCacheControl(const char* cache_control);
     AsyncStaticWebHandler& setLastModified(const char* last_modified);
     AsyncStaticWebHandler& setLastModified(struct tm* last_modified);
+    AsyncStaticWebHandler& setLastModified(time_t last_modified);
+    AsyncStaticWebHandler& setLastModified(); //sets to current time. Make sure sntp is runing and time is updated
 };
 
 class AsyncCallbackWebHandler: public AsyncWebHandler {

src/WebHandlers.cpp

@@ -68,6 +68,17 @@ AsyncStaticWebHandler& AsyncStaticWebHandler::setLastModified(struct tm* last_mo
   return setLastModified((const char *)result);
 }
 
+AsyncStaticWebHandler& AsyncStaticWebHandler::setLastModified(time_t last_modified){
+  return setLastModified((struct tm *)gmtime(&last_modified));
+}
+
+AsyncStaticWebHandler& AsyncStaticWebHandler::setLastModified(){
+  time_t last_modified;
+  if(time(&last_modified) == 0) //time is not yet set
+    return *this;
+  return setLastModified(last_modified);
+}
+
 bool AsyncStaticWebHandler::canHandle(AsyncWebServerRequest *request)
 {
   if (request->method() == HTTP_GET &&