[Firebase Admob] Add more AnchorTypes, AnchorType.topRight,AnchorType.topLeft ,etc

[elMuso]

Right now we have horizontalCenterOffset to horizontally position an ad, however it's not fit for a use case. The layout that i have requires the ad to be always in the top right , so it would be awesome if the anchor would consider horizontal values like right and left. I know that in android we use Gravity for the anchor, so the behaviour should be possible via
content.setGravity(verticalAnchorType|horizontalAnchorType); in MobileAd.java.
But for Ios i'm lost, i'm not that familiar with how ios works. It wouls be a nice feature ;)