Provide a more straightforward method to implement custom InAppMessage view

## What feature would you like to see?

Would be great to have a possibility to implement only a custom `InAppMessage` to `View` mapper without a need to build all the machinery required by `FirebaseInAppMessagingDisplay`. Something similar to `RemoteViews`

## How would you use it?

there would be just a callback needed to be implemented, something like:
```
fun getView(inAppMessage: InAppMessage, someContext: ...): View
```
which would return a view displayed by the firebase SDK in a piece of UI managed by it.