App distribution new build alerts sdk (#5204)

* Initial commit for Firebase App Distribution New Build Alerts SDK 
* Added public header and implemented the public methods
* Added App Delegate swizzling for the App Auth library


Co-authored-by: Jeremy Durham <jeremydurham@google.com>

FirebaseAppDistribution.podspec

@@ -0,0 +1,43 @@
+Pod::Spec.new do |s|
+  s.name             = 'FirebaseAppDistribution'
+  s.version          = '0.0.1'
+  s.summary          = 'App Distribution for Firebase iOS SDK.'
+
+  s.description      = <<-DESC
+iOS SDK for App Distribution for Firebase.
+                       DESC
+
+  s.homepage         = 'https://developers.google.com/'
+  s.license          = { :type => 'Apache', :file => 'LICENSE' }
+  s.authors          = 'Google, Inc.'
+  s.source           = {
+    :git => 'https://github.com/firebase/firebase-ios-sdk.git',
+    :tag => 'AppDistribution-' + s.version.to_s
+  }
+
+  s.ios.deployment_target = '8.0'
+
+  s.cocoapods_version = '>= 1.4.0'
+  s.static_framework = true
+  s.prefix_header_file = false
+
+  base_dir = "FirebaseAppDistribution/Sources/"
+  s.source_files = base_dir + '**/*.{c,h,m,mm}'
+  s.public_header_files = base_dir + 'Public/*.h'
+  s.private_header_files = base_dir + 'Private/*.h'
+
+  s.dependency 'FirebaseCore', '~> 6.6'
+  s.dependency 'AppAuth', '~> 1.2.0'
+  s.dependency 'GoogleUtilities/AppDelegateSwizzler', '~> 6.5'
+
+  s.pod_target_xcconfig = {
+    'GCC_C_LANGUAGE_STANDARD' => 'c99',
+    'GCC_PREPROCESSOR_DEFINITIONS' => 'FIRAppDistribution_VERSION=' + s.version.to_s
+  }
+
+  s.test_spec 'unit' do |unit_tests|
+   unit_tests.source_files = 'FirebaseAppDistribution/Tests/Unit*/*.[mh]'
+  end
+
+  # end
+end