Add runner management health check for /ping

Even though we can check the health of the runner management, we would like to ensure an external health check validates the availability of all required services.

lib/runner/strategy.rb

@@ -53,6 +53,10 @@ class Runner::Strategy
     raise NotImplementedError
   end
 
+  def self.health
+    raise NotImplementedError
+  end
+
   def self.release
     raise NotImplementedError
   end