Add Auth interoperability library. (#1501)

* Add Auth interoperability library.

This allows other SDKs to retrieve the user ID and fetch a new
Auth token in a type-safe way through the Core container.

* Remove unnecessary import.

* Add missing copyright.

* Resolve comments.

- Remove unnecessary umbrella header.
- Move to version 1.0.
- Add tvOS support for app lifecycle changes.

* Move public interop headers to private.

* Add public_header_files too.

* Moved headers back into public.

* Shorten Interoperability and Interoperable to Interop.

* Add AuthInterop to travis linting.

* Fixed tag format in AuthInterop.podspec.

* Add AuthInterop path to Firestore Podfile.

FirebaseAuth.podspec

@@ -61,6 +61,7 @@ supports email and password accounts, as well as several 3rd party authenticatio
   }
   s.framework = 'SafariServices'
   s.framework = 'Security'
+  s.dependency 'FirebaseAuthInterop', '~> 1.0'
   s.dependency 'FirebaseCore', '~> 5.0'
   s.dependency 'GoogleUtilities/Environment', '~> 5.0'
   s.dependency 'GTMSessionFetcher/Core', '~> 1.1'