feat: Builder for creating Campaign conversations (#2192)

- Builder for creating Campaign conversations
- Widget endpoint to fetch the campaigns

spec/builders/campaigns/campaign_conversation_builder_spec.rb

@@ -0,0 +1,31 @@
+require 'rails_helper'
+
+describe ::Campaigns::CampaignConversationBuilder do
+  let(:account) { create(:account) }
+  let(:inbox) { create(:inbox, account: account) }
+  let(:contact) { create(:contact, account: account, identifier: '123') }
+  let(:contact_inbox) { create(:contact_inbox, contact: contact, inbox: inbox) }
+  let(:campaign) { create(:campaign, inbox: inbox, account: account) }
+
+  describe '#perform' do
+    it 'creates a conversation with campaign id and message with campaign message' do
+      campaign_conversation = described_class.new(
+        contact_inbox_id: contact_inbox.id,
+        campaign_display_id: campaign.display_id
+      ).perform
+
+      expect(campaign_conversation.campaign_id).to eq(campaign.id)
+      expect(campaign_conversation.messages.first.content).to eq(campaign.message)
+    end
+
+    it 'will not create a conversation with campaign id if another conversation exists' do
+      create(:conversation, contact_inbox_id: contact_inbox.id, inbox: inbox, account: account)
+      campaign_conversation = described_class.new(
+        contact_inbox_id: contact_inbox.id,
+        campaign_display_id: campaign.display_id
+      ).perform
+
+      expect(campaign_conversation).to eq(nil)
+    end
+  end
+end