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.

test/configuration_test.rb

@@ -344,4 +344,12 @@ class ConfigurationTest < ActiveSupport::TestCase
 
     assert_raises(Kamal::ConfigurationError) { Kamal::Configuration.new(@deploy_with_roles.merge(retain_containers: 0)) }
   end
+
+  test "extensions" do
+    dest_config_file = Pathname.new(File.expand_path("fixtures/deploy_with_extensions.yml", __dir__))
+
+    config = Kamal::Configuration.create_from config_file: dest_config_file
+    assert_equal config.role(:web_tokyo).running_traefik?, true
+    assert_equal config.role(:web_chicago).running_traefik?, true
+  end
 end