set real "Last-Modified" header based on file's LastWrite time

Get file's LastWrite timestamp for file handlers (if supported by FS driver)
and construct proper "Last-Modified" header. Works fine for LittleFS.
If not supported by FS than fallback to previous implementation with manual value for "Last-Modified".

an example code to serve static files from LittleFS with IMS revalidation
```
// serve all static files from LittleFS root /
server.serveStatic("/", LittleFS, "/")
        .setDefaultFile("index.html")
        .setCacheControl("must-revalidate");  // revalidate based on etag/IMS headers
```

Signed-off-by: Emil Muratov <[email protected]>

src/WebHandlerImpl.h

@@ -28,6 +28,7 @@
 
 #include "stddef.h"
 #include <time.h>
+#include <ctime>
 
 class AsyncStaticWebHandler: public AsyncWebHandler {
    using File = fs::File;
@@ -55,7 +56,7 @@ class AsyncStaticWebHandler: public AsyncWebHandler {
     AsyncStaticWebHandler& setDefaultFile(const char* filename);
     AsyncStaticWebHandler& setCacheControl(const char* cache_control);
     AsyncStaticWebHandler& setLastModified(const char* last_modified);
-    AsyncStaticWebHandler& setLastModified(struct tm* last_modified);
+    AsyncStaticWebHandler& setLastModified(const std::tm* last_modified);
   #ifdef ESP8266
     AsyncStaticWebHandler& setLastModified(time_t last_modified);
     AsyncStaticWebHandler& setLastModified(); //sets to current time. Make sure sntp is runing and time is updated

src/WebHandlers.cpp

@@ -58,18 +58,15 @@ AsyncStaticWebHandler& AsyncStaticWebHandler::setCacheControl(const char* cache_
 }
 
 AsyncStaticWebHandler& AsyncStaticWebHandler::setLastModified(const char* last_modified){
-  _last_modified = String(last_modified);
+  _last_modified = last_modified;
   return *this;
 }
 
-AsyncStaticWebHandler& AsyncStaticWebHandler::setLastModified(struct tm* last_modified){
-  auto formatP = PSTR("%a, %d %b %Y %H:%M:%S %Z");
-  char format[strlen_P(formatP) + 1];
-  strcpy_P(format, formatP);
-
-  char result[30];
-  strftime(result, sizeof(result), format, last_modified);
-  return setLastModified((const char *)result);
+AsyncStaticWebHandler& AsyncStaticWebHandler::setLastModified(const std::tm* last_modified){
+  constexpr size_t buffsize = sizeof("Fri, 27 Jan 2023 15:50:27 GMT");    // a format for LM header
+  char result[buffsize];
+  std::strftime(result, buffsize, "%a, %d %b %Y %H:%M:%S GMT", last_modified);
+  return setLastModified(static_cast<const char *>(result));
 }
 
 #ifdef ESP8266
@@ -198,14 +195,16 @@ uint8_t AsyncStaticWebHandler::_countBits(const uint8_t value) const
 void AsyncStaticWebHandler::handleRequest(AsyncWebServerRequest *request)
 {
   // Get the filename from request->_tempObject and free it
-  String filename = String((char*)request->_tempObject);
+  String filename((char*)request->_tempObject);
   free(request->_tempObject);
   request->_tempObject = NULL;
   if((_username.length() && _password.length()) && !request->authenticate(_username.c_str(), _password.c_str()))
       return request->requestAuthentication();
 
   if (request->_tempFile == true) {
-    String etag = String(request->_tempFile.size());
+    time_t lw = request->_tempFile.getLastWrite();    // get last file mod time (if supported by FS)
+    if (lw) setLastModified(std::gmtime(&lw));
+    String etag(lw ? lw : request->_tempFile.size());   // set etag to lastmod timestamp if available, otherwise to size
     if (_last_modified.length() && _last_modified == request->header(F("If-Modified-Since"))) {
       request->_tempFile.close();
       request->send(304); // Not modified