Use securekey-textarea widget for SA json fields

[GnsP]

Changes the widget type for SA json fields from textbox to secure-textarea

Please notice the shield icon in the SA json field. Clicking the shield lets the user select one of the secure keys defined in the namespace.

A corresponding PR in cdap-ui adds the securekeys tab in Namespace Admin and also makes the securekey-textarea widget non-editable (so that the user will only be able to add the value as a secure macro).