Add support or update docs for using sending custom_data

[Squidy06]

Hello,
According to your docs here:
https://developers.heyzap.com/docs/advanced-publishing#serverside-rewarded-video-callbacks

I could send the user_id from my mobile app when showing an ad, and then receive this id from your server-side callbacks. Something like "showWithOptions"? Should be in this sdk version right?

But how to use it with this cordova plugin? I can't see such option in latest docs.

Best regards :)

[ezpuzz]

this function is only supported on iOS native, Android native and Unity right now.