actionmailer: Add ActionMailer::Base#default_i18n_subject and #headers

refs:

* https://api.rubyonrails.org/classes/ActionMailer/Base.html#method-i-default_i18n_subject
* https://api.rubyonrails.org/classes/ActionMailer/Base.html#method-i-headers

Author: tk0miya

gems/actionmailer/7.0/_test/railsguides.rb

@@ -27,6 +27,12 @@ def welcome_email
     end
   end
 
+  class I18nUserMailer < ActionMailer::Base
+    def welcome(email, username)
+      mail(to: email, subject: default_i18n_subject(user: username))
+    end
+  end
+
   NotifierMailer.welcome("first user").deliver_now
   NotifierMailer.welcome("first user").message
   NotifierMailer.welcome("first user").deliver_later

gems/actionmailer/7.0/_test/railsguides.rbs

@@ -13,7 +13,11 @@ module RailsGuides
   class UserMailer < ApplicationMailer
     @user: untyped
     @url: String
-    
+
     def welcome_email: () -> Mail::Message
   end
+
+  class I18nUserMailer < ApplicationMailer
+    def welcome: (String email, String username) -> Mail::Message
+  end
 end

gems/actionmailer/7.0/actionmailer.rbs

@@ -24,6 +24,13 @@ module ActionMailer
     def params: () -> Hash[untyped, untyped]
 
     def self?.email_address_with_name: (String, String) -> String
+
+    def headers: () -> Mail::Message
+               | (Hash[untyped, untyped]) -> untyped
+
+    private
+
+    def default_i18n_subject: (?Hash[untyped, untyped] interpolations) -> String
   end
 
   class MessageDelivery