Define DryRunnable#dry_run?

A dry-runnable Agent can use this method to decide if it should avoid
affecting unsandboxed resources, like calling an external API that
might change any state.

app/concerns/dry_runnable.rb

@@ -25,6 +25,10 @@ module DryRunnable
     )
   end
 
+  def dry_run?
+    is_a? Sandbox
+  end
+
   module Sandbox
     attr_accessor :results