FEATURE: Log the SMTP response in EmailLog (#17056)

When sending emails with delivery_method_options -> return_response
set to true, the SMTP sending code inside Mail will return the SMTP
response when calling deliver! for mail within the app. This commit
ensures that Email::Sender captures this response if it is returned
and stores it against the EmailLog created for the sent email.

A follow up PR will make this visible within the admin email UI.

db/migrate/db/migrate/20220606061813_add_smtp_transaction_response_to_email_logs.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddSmtpTransactionResponseToEmailLogs < ActiveRecord::Migration[7.0]
+  def change
+    add_column :email_logs, :smtp_transaction_response, :string, null: true, limit: 500
+  end
+end