Behaviour: Support Sprint field#147

As a user, I would like to be able to use Sprint field in Behaviour, so that I can create some condition logic to make the field as readonly/hidden based on the current user’s group.

Same need than @Tiong Lih Yao , I need to set Sprint as read-only but based on a certain status.
By the way @Tiong Lih Yao , you should be able to achieve something like that with a permission scheme that allow to “Schedule issues” for some of your user’s groups.