feat: new configuration system (and schema) (#2819)

script/test_app_smtp.rb

@@ -25,9 +25,9 @@ rescue StandardError => e
   puts "\e[31mMessage was not delivered successfully to SMTP server.\e[0m"
   puts "Error: #{e.class} (#{e.message})"
   puts
-  puts "  SMTP Host: #{Postal.config.smtp.host}"
-  puts "  SMTP Port: #{Postal.config.smtp.port}"
-  puts "  SMTP Username: #{Postal.config.smtp.username}"
-  puts "  SMTP Password: #{Postal.config.smtp.password}"
+  puts "  SMTP Host: #{Postal::Config.smtp.host}"
+  puts "  SMTP Port: #{Postal::Config.smtp.port}"
+  puts "  SMTP Username: #{Postal::Config.smtp.username}"
+  puts "  SMTP Password: #{Postal::Config.smtp.password}"
   puts
 end