Register autoload for ActiveJob::Arguments

Since #50715, there are cases where `ActiveJob::Arguments` is used
but not required anymore in the test environment. This could be a
test for a custom argument serializer.

One solution could be to add a require "active_job/arguments" in
all the usages, but that isn't a convention we follow in Rails.

Instead, let's register the autoload for `ActiveJob::Arguments` in
`active_job.rb` so that it's always available when needed.

actionmailer/test/mailers/delayed_mailer.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "active_job/arguments"
-
 class DelayedMailerError < StandardError; end
 
 class DelayedMailer < ActionMailer::Base

activejob/lib/active_job.rb

@@ -36,6 +36,9 @@ module ActiveJob
 
   autoload :Base
   autoload :QueueAdapters
+  autoload :Arguments
+  autoload :DeserializationError, "active_job/arguments"
+  autoload :SerializationError, "active_job/arguments"
 
   eager_autoload do
     autoload :Serializers

activejob/lib/active_job/enqueuing.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "active_job/arguments"
-
 module ActiveJob
   # Provides behavior for enqueuing jobs.
 

activejob/lib/active_job/execution.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require "active_support/rescuable"
-require "active_job/arguments"
 
 module ActiveJob
   # = Active Job \Execution

activejob/test/cases/argument_serialization_test.rb

@@ -3,7 +3,6 @@
 require "json"
 require "bigdecimal"
 require "helper"
-require "active_job/arguments"
 require "models/person"
 require "active_support/core_ext/hash/indifferent_access"
 require "active_support/core_ext/integer/time"