OTA: Add firmware update, file remove callbacks

[VitaliyAndreevich]

Ussage example:

sett.onUpdateFWStart([]() {
  Serial.printf("Update started\n\r");
});

sett.onUpdateFWProgress([](size_t current, size_t final) {
  size_t progress= (100 * current) / final;
  Serial.printf("update %d\n\r", progress);
});

sett.onUpdateFWDone([](bool res) {
  Serial.printf("Update done. res = %d\n\r", res);
});

sett.onFileRemove([](Text file_name) {
  Serial.printf("Remove file <%s>\n\r", file_name.toString().c_str());
});