Introduce after_register hook

Allows you to take action when a new key is registered, so that you can
decorate the key without knowing its name ahead of time.

See also hanami/hanami#1360

Author: alassek

lib/dry/system/container.rb

@@ -486,6 +486,15 @@ def root
           config.root
         end
 
+        # @api public
+        def register(key, *)
+          super
+
+          hooks[:after_register].each { |hook| instance_exec(key, &hook) }
+
+          self
+        end
+
         # @api public
         def resolve(key)
           load_component(key) unless finalized?

spec/unit/container/hooks/after_hooks_spec.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+RSpec.describe Dry::System::Container do
+  subject(:system) do
+    Class.new(described_class)
+  end
+
+  describe "after_register hook" do
+    it "executes after a new key is registered" do
+      expect { |hook|
+        system.after(:register, &hook)
+        system.register(:foo) { "bar" }
+      }.to yield_with_args(:foo)
+    end
+  end
+end