Feat: Add support for Collapsible banner ads

[NNBnh]

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

I want this https://developers.google.com/ad-manager/mobile-ads-sdk/android/banner/collapsible in my app please.

Describe alternatives you've considered

There are no alts, my boss told me to add this :(.

[distante]

Your best bet here is to clone the repository and ad the implementation. Looks doable.