feat: Builder for creating Campaign conversations (#2192)

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

app/controllers/api/v1/widget/campaigns_controller.rb

@@ -0,0 +1,13 @@
+class Api::V1::Widget::CampaignsController < Api::V1::Widget::BaseController
+  skip_before_action :set_contact
+
+  def index
+    @campaigns = @web_widget.inbox.campaigns.where(enabled: true)
+  end
+
+  private
+
+  def permitted_params
+    params.permit(:website_token)
+  end
+end