[Admob] Adaptive Banners

[JeffGuKang]

https://developers.google.com/admob/ios/banner/adaptive

They recommend to use Adaptive Banners instead of Smart Banners.

[dylancom]

Would like to see this integrated aswell! This is the new generation of mobile banners.

[mikehardy]

@dylancom we will happily accept and merge PRs - this not something that is a maintainer priority (and I personally believe AdMob should actually be peeled out of firebase since there are literally zero firebase-specific admob APIs) but if someone from the community contributes a clean PR, I'll shepherd it to release

[dylancom]

@mikehardy I understand. I just submitted a PR that adds adaptive banner support on iOS. Might need some help to get it also to work on Android.