SD: Implement readBytes (#4931)

This speeds up the ESP8266WebServer::streamFile more than 3 times. Tested on streaming the 800+ Kb file from SD (FAT32), average time without a fix was 9000 ms, with the fix is 2600 ms (maximal possible SPI speed used), which is as fast as streaming the same file from internal SPIFFS. Hardware: WeMos D1 mini.

libraries/SD/src/File.cpp

@@ -95,6 +95,11 @@ int File::read(void *buf, uint16_t nbyte) {
   return 0;
 }
 
+size_t File::readBytes(char *buffer, size_t length) {
+  int result = read(buffer, (uint16_t)length);
+  return result < 0 ? 0 : (size_t)result;
+}
+
 int File::available() {
   if (! _file) return 0;