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.

FirebaseAuthInterop.podspec

@@ -0,0 +1,29 @@
+Pod::Spec.new do |s|
+  s.name             = 'FirebaseAuthInterop'
+  s.version          = '1.0.0'
+  s.summary          = 'Interfaces that allow other Firebase SDKs to use Auth functionality.'
+
+  s.description      = <<-DESC
+  INTERNAL ONLY: A set of protocols that other Firebase SDKs can use to interoperate with Auth in a
+  safe and reliable manner.
+                       DESC
+
+  s.homepage         = 'https://firebase.google.com'
+  s.license          = { :type => 'Apache', :file => 'LICENSE' }
+  s.authors          = 'Google, Inc.'
+
+  # NOTE that these should not be used externally, this is for Firebase pods to depend on each
+  # other.
+  s.source           = {
+    :git => 'https://github.com/firebase/firebase-ios-sdk.git',
+# TODO: Remove this once it is merged in master and ready for release in M30.
+# :tag => 'AuthInterop-' + s.version.to_s
+    :tag => 'pre-AuthInterop-' + s.version.to_s
+  }
+  s.social_media_url = 'https://twitter.com/Firebase'
+  s.ios.deployment_target = '8.0'
+  s.osx.deployment_target = '10.10'
+  s.tvos.deployment_target = '10.0'
+  s.source_files = 'Interop/Auth/**/*.h'
+  s.public_header_files = 'Interop/Auth/Public/*.h'
+end