Add option to download only on wifi

First of all, thank you for this awesome package.

I want the user to be able to set an option so that the download doesn't happen in the background using the mobile data.

Scenario:
1. User without an internet connection / while using mobile data opens the app.
2. I want to enqueue the download file, but I don't want it to start downloading (if the user is on mobile data).
3. Now the user is done using the app and force closes the app, and after some time user gets connected to the wifi.
4. Only then I want download to happen, (currently, it start downloading even on the mobile data, as expected)

So, can we have an additional parameter for `FlutterDownloader.enqueue()` for e.g `bool` field `downloadOnlyOnWifi`.

If there is another workaround for this that would be great as well.

Many thanks.