Add a getter for clickUrl inside the PrebidNativeAd class.

[eroncastro]

Is your feature request related to a problem? Please describe.

Me and my team are integrating Prebid Mobile with Google Mobile Ads.
Right now, we are trying to use it in an Android TV Device. Right now, we were unable to obtain the property clickUrl from PrebidNativeAd, because it is private. We need it to generate a QR code from it and make it "clickable" when accessed from a TV.

Describe the solution you'd like
Add a getter for clickUrl inside the PrebidNativeAd class, making the property accessible and allowing its use for a QR generator.

Describe alternatives you've considered
None.

Additional context
clickUrl property