title: Validate params based on condition in rails-grape
body: <p>I am developing APIs for a web app. In one specific case I am facing a situation where I require param2 only when value of param1 is false. I was able to use conditional statements to achieve it. I would like to know is there any inbuilt methods in grape for this type of validation. Something on the lines below</p>
  
  <pre><code>params do
   requires :model_name, type: Hash do
     optional :params1
     if params1 == false
       require :params2
     end
   end
  end
  </code></pre>