Extract Kamal::Configuration::Proxy::Boot

This is for boot time configuration for the kamal proxy. Config in here
doesn't not belong in Kamal::Configuration::Proxy which is for deploy
time configuration for the app itself.

Kamal apps don't contain boot time config, because multiple apps can
share a proxy and the config could conflict.

lib/kamal/configuration/proxy.rb

@@ -74,6 +74,6 @@ class Kamal::Configuration::Proxy
     end
 
     def error_pages
-      File.join config.proxy_error_pages_container_directory, config.version if config.error_pages_path
+      File.join config.proxy_boot.error_pages_container_directory, config.version if config.error_pages_path
     end
 end