Add scaffold of EE extensions to CI/CD bridge jobs

app/models/ci/bridge.rb

@@ -32,3 +32,5 @@ module Ci
     end
   end
 end
+
+::Ci::Bridge.prepend(::EE::Ci::Bridge)

ee/app/models/ee/ci/bridge.rb

+# frozen_string_literal: true
+
+module EE
+  module Ci
+    module Bridge
+      extend ActiveSupport::Concern
+
+      prepended do
+        has_many :sourced_pipelines, class_name: ::Ci::Sources::Pipeline,
+                                     foreign_key: :source_job_id
+
+        state_machine :status do
+          after_transition any => [:pending] do |bridge|
+            bridge.run_after_commit do
+              bridge.schedule_downstream_pipeline!
+
+              # 1. schedule pipeline creation async
+              # 2. scheduled pipeline calls-back to change state to running
+              #    when it gets created successfully
+              # 3. if no downstream pipeline can not be created because of
+              #    various reasons like lack of access then we change state to
+              #    failed with a reason
+            end
+          end
+
+          after_transition pending: :running do |bridge|
+            bridge.run_after_commit do
+            end
+          end
+        end
+      end
+
+      def schedule_downstream_pipeline!
+        raise NotImplementedError
+      end
+    end
+  end
+end

ee/spec/models/ci/bridge_spec.rb

+require 'spec_helper'
+
+describe Ci::Bridge do
+  set(:project) { create(:project) }
+  set(:pipeline) { create(:ci_pipeline, project: project) }
+
+  let(:bridge) do
+    create(:ci_bridge, status: :created, pipeline: pipeline)
+  end
+
+  it 'has many sourced pipelines' do
+    expect(bridge).to have_many(:sourced_pipelines)
+  end
+
+  describe 'state machine transitions' do
+    context 'when it changes status from created to pending' do
+      it 'schedules downstream pipeline creation' do
+        expect(bridge).to receive(:schedule_downstream_pipeline!)
+
+        bridge.enqueue!
+      end
+    end
+  end
+end