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.rb

@@ -47,7 +47,7 @@ class Kamal::Configuration
     @destination = destination
     @declared_version = version
 
-    validate! raw_config, example: validation_yml.symbolize_keys, context: ""
+    validate! raw_config, example: validation_yml.symbolize_keys, context: "", with: Kamal::Configuration::Validator::Configuration
 
     # Eager load config to validate it, these are first as they have dependencies later on
     @servers = Servers.new(config: self)