Provide a more straightforward method to implement custom InAppMessage view

[mszerling-ynd]

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.

[google-oss-bot]

I couldn't figure out how to label this issue, so I've labeled it for a human to triage. Hang tight.