title: text_field_tag with ActiveAdmin
body: <p>I've a hard time with ActiveAdmin and their DSL. I'm using it to build my admin and at some point in a form I need to have a <code>text_field_tag</code>; I mean some fields which aren't related to the model I'm manipulating in the form which will be sent through with the model related data.</p>
  
  <p>A custom text field basically. Nothing too crazy.</p>
  
  <p>So I've built this</p>
  
  <pre><code>panel 'Send payment authorization' do
    active_admin_form_for EventPaymentAuthorization.new, url: { action: :send_event_payment_authorization } do |f|
      f.inputs do
        f.input :body, as: :text
  
        f.text_field_tag :line_items_label
        f.text_field_tag :line_items_amount
  
        f.input :fees_in_cents, as: :select, collection: [:free, :automatic], prompt: true, selected: :automatic
      end
      f.actions do
        f.action :submit, label: 'Create payment authorization'
      end
    end
  end
  </code></pre>
  
  <p>The <code>f.text_field_tag</code> get simply ignored by ActiveAdmin. Why is that? It doesn't raise any error, but it doesn't show either.</p>
  
  <p>The reason I need custom unrelated inputs is because <code>line_items</code> in my example is a JSONB with values such as <code>[{amount: 0.0, label: 'Hello'}]</code> and I don't believe it can be processed through Formtastic or ActiveAdmin natively. It's also always good to be able to create  custom inputs when sending data.</p>