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/docs/configuration.yml

@@ -2,13 +2,24 @@
 #
 # Configuration is read from the `config/deploy.yml`
 #
+
+# Destinations
+#
 # When running commands, you can specify a destination with the `-d` flag,
 # e.g. `kamal deploy -d staging`
 #
 # In this case the configuration will also be read from `config/deploy.staging.yml`
 # and merged with the base configuration.
+
+# Extensions
 #
-# The available configuration options are explained below.
+# Kamal will not accept unrecognized keys in the configuration file.
+#
+# However, you might want to declare a configuration block using YAML anchors
+# and aliases to avoid repetition.
+#
+# You can use prefix a configuration section with `x-` to indicate that it is an
+# extension. Kamal will ignore the extension and not raise an error.
 
 # The service name
 # This is a required value. It is used as the container name prefix.