Add Buildkite step to optionally trigger FAD build on trunk (#16432)

Author: mokagio

.buildkite/pipeline.yml

@@ -31,6 +31,20 @@ steps:
       - github_commit_status:
           context: Prototype Build
 
+  - group: ":rocket: Prototype Build"
+    if: build.branch == "trunk"
+    steps:
+      - input: Deploy Prototype Build?
+        prompt: Share a Prototype Build via Firebase App Distribution?
+        key: prototype_triggered
+      - label: Prototype Build
+        depends_on: prototype_triggered
+        command: .buildkite/commands/prototype-build.sh
+        plugins: [$CI_TOOLKIT]
+        notify:
+          - github_commit_status:
+              context: Prototype Build From Trunk
+
   #################
   # Run Unit Tests
   #################

fastlane/Fastfile

@@ -29,6 +29,9 @@ WORKSPACE_PATH = File.join(PROJECT_ROOT_FOLDER, 'WooCommerce.xcworkspace')
 FIREBASE_APP_ID = '1:124902176124:ios:02259de1e7c42b291620f9'
 FIREBASE_TESTERS_GROUP = 'woocommerce-ios---prototype-builds'
 
+# Buildkite sets this env var to the PR number if on a PR, but to 'false' (and not nil) if not on a PR
+pull_request_number = ENV['BUILDKITE_PULL_REQUEST']&.then { |n| n == 'false' ? nil : Integer(n) }
+
 BUILDKITE_RELEASE_PIPELINE = 'release-builds.yml'
 IOS_LOCALES = %w[ar-SA de-DE en-US es-ES fr-FR he id it ja ko nl-NL pt-BR ru sv tr zh-Hans zh-Hant].freeze
 SIMULATOR_VERSION = '18.5' # For screenshots
@@ -777,10 +780,10 @@ platform :ios do
     build_for_prototype_build
 
     release_notes = <<~NOTES
-      Pull Request: ##{pull_request_number || 'N/A'}
       Branch: `#{ENV.fetch('BUILDKITE_BRANCH', 'N/A')}`
       Commit: #{ENV.fetch('BUILDKITE_COMMIT', 'N/A')[0...7]}
     NOTES
+    release_notes += "Pull Request: ##{pull_request_number}" unless pull_request_number.nil?
 
     firebase_app_distribution(
       app: FIREBASE_APP_ID,
@@ -1526,12 +1529,6 @@ TEST_ANALYTICS_ENVIRONMENT = %w[
 # Release Management Utils
 # -----------------------------------------------------------------------------------
 
-def pull_request_number
-  # Buildkite sets this env var to the PR number if on a PR, but to 'false' (and not nil) if not on a PR
-  pr_num = ENV.fetch('BUILDKITE_PULL_REQUEST', 'false')
-  pr_num == 'false' ? nil : Integer(pr_num)
-end
-
 def create_backmerge_pr
   version = release_version_current