Add support for configuration extensions

Allow blocks prefixed with `x-` in the configuration as a place to
declare reusable blocks with YAML anchors and aliases.

Borrowed from the Docker Compose configuration file format -
https://github.com/compose-spec/compose-spec/blob/main/spec.md#extension

Thanks to @ruyrocha for the suggestion.

lib/kamal/configuration/validator/configuration.rb

@@ -0,0 +1,6 @@
+class Kamal::Configuration::Validator::Configuration < Kamal::Configuration::Validator
+  private
+    def allow_extensions?
+      true
+    end
+end