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

test/fixtures/deploy_with_extensions.yml

@@ -0,0 +1,24 @@
+
+x-web: &web
+  traefik: true
+
+service: app
+image: dhh/app
+servers:
+  web_chicago:
+    <<: *web
+    hosts:
+      - 1.1.1.1
+      - 1.1.1.2
+  web_tokyo:
+    <<: *web
+    hosts:
+      - 1.1.1.3
+      - 1.1.1.4
+env:
+  REDIS_URL: redis://x/y
+registry:
+  server: registry.digitalocean.com
+  username: user
+  password: pw
+primary_role: web_tokyo