Ability for Computed Attributes to use XDM fields from a schema relationship

Currently, the Computed Attribute feature does not support fields based on the `relationship` XDM construct.

If using one of those, you will get a 500 error upon publishing

Why is this feature important to you:

Ability to use relationship exists at schema level, you'd want to be able to leverage this across the product, including when building Computed Attribute for added flexibility.