Add progress callback to Update::writeStream().

[everslick]

This is a more or less verbatim port from the ArduinoOTA library to the UpdateClass. Usage example:

  Update.onProgress(
    [](size_t current, size_t total) {
      Serial.printf("\r  received %d of %d bytes           ", current, total);
    }
  );

Very useful for possible inclusion in the HttpUpdate library.

[me-no-dev]

nice!