[API Request] new progress API for ESPhttpUpdate download + upload ?

Would it be possible to have two new APIs for `ESPhttpUpdate` class to notify user code for download + upload progress?
Like this:
```cpp
using prog_cb_t = std::function<void(size_t total, size_t got /* or written */)>();

ESP8266HTTPUpdate::onDownloadProgress(prog_cb_t);
ESP8266HTTPUpdate::onUploadProgress(prog_cb_t)
```