v1.6.0 to save heap when sending large data

### Releases v1.6.0

1. Support using `CString` to save heap to send `very large data`. Check [request->send(200, textPlainStr, jsonChartDataCharStr); - Without using String Class - to save heap #8](khoih-prog/Portenta_H7_AsyncWebServer#8)
2. Add functions and example `Async_AdvancedWebServer_favicon` to support `favicon.ico`
3. Add multiple examples to demo the new feature
4. Fix issue with slow browsers or network
5. Change license from `MIT` to `GPLv3` to match with original [ESPAsyncWebServer](https://github.com/me-no-dev/ESPAsyncWebServer) license