Add ActionMailbox.deprecator

This commit adds `ActionMailbox.deprecator`, and adds it to
`Rails.application.deprecators` so that it can be configured via
settings such as `config.active_support.report_deprecations`.

actionmailbox/lib/action_mailbox.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "action_mailbox/deprecator"
 require "action_mailbox/mail_ext"
 
 module ActionMailbox

actionmailbox/lib/action_mailbox/deprecator.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+module ActionMailbox
+  def self.deprecator # :nodoc:
+    @deprecator ||= ActiveSupport::Deprecation.new
+  end
+end

actionmailbox/lib/action_mailbox/engine.rb

@@ -22,6 +22,10 @@ module ActionMailbox
 
     config.action_mailbox.storage_service = nil
 
+    initializer "action_mailbox.deprecator" do |app|
+      app.deprecators[:action_mailbox] = ActionMailbox.deprecator
+    end
+
     initializer "action_mailbox.config" do
       config.after_initialize do |app|
         ActionMailbox.logger = app.config.action_mailbox.logger || Rails.logger

railties/test/application/configuration_test.rb

@@ -3893,6 +3893,7 @@ module ApplicationTests
       assert_equal AbstractController.deprecator, Rails.application.deprecators[:action_controller]
       assert_equal ActionController.deprecator, Rails.application.deprecators[:action_controller]
       assert_equal ActionDispatch.deprecator, Rails.application.deprecators[:action_dispatch]
+      assert_equal ActionMailbox.deprecator, Rails.application.deprecators[:action_mailbox]
       assert_equal ActionMailer.deprecator, Rails.application.deprecators[:action_mailer]
       assert_equal ActionView.deprecator, Rails.application.deprecators[:action_view]
       assert_equal ActiveJob.deprecator, Rails.application.deprecators[:active_job]