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

config/boot.rb

@@ -5,6 +5,5 @@ ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../Gemfile", __dir__)
 require "bundler/setup" # Set up gems listed in the Gemfile.
 
 require_relative "../lib/postal/config"
-Postal.check_config!
 
-ENV["RAILS_ENV"] = Postal.config.rails&.environment || "development"
+ENV["RAILS_ENV"] = Postal::Config.rails.environment || "development"