Add IterableInAppMessage to index.ts

[16francej]

The type IterableInAppMessage should be accessible from the top-level index.ts file. It's necessary for typescript users who are looking to overwrite the inAppHandler per https://support.iterable.com/hc/en-us/articles/360045714172-In-App-Messages-with-Iterable-s-React-Native-SDK-#showing-or-skipping-an-incoming-in-app-message. This is a common use-case, and should be available through the same import as Iterable and IterableConfig.

Also, are permissions necessary to push to a branch in this repo? I didn't see anything in contributing.md, but when I tried to push to a branch I received a permission denied error.

Thanks.