firebase-ios-sdk

Firebase In-app messaging callbacks (#2354)

* FIRInAppMessagingDisplayMessage and FIRInAppMessagingDisplayDelegate refactor + downstream changes

* More downstream changes from FIRInAppMessagingDisplayDelegate, calling through to app-side delegate from default display executor

* Add delegate property on FIRInAppMessaging

* Display SDK changes to pass through in app message with callbacks

* Headless SDK unit test refactoring

* Display test app downstream changes

* Pull in headless SDK in display app, no need to have this come in as a framework anymore

* Run scripts/style.sh

* Remove WIP test

* Refactor FIRIAMDIsplayExecutor to take an InAppMessaging object for testing

* Depend on FirebaseAnalyticsInterop in UI test app

* File imports for FIRInAppMessaging files

* Remove FirebaseInAppMessaging imports