Relationship data controller helper methods

[nicklandgrebe]

Story

As a developer, I want to be able to access relationship data, like for /data/relationships/transactions/data, but like this:

transactions_data # returns nil
transactions_data! # fails with error

Or nested associations like

transactions_payment_method_data
transactions_payment_method_data!

Return overridden object that allows you to retrieve the object referenced in the data

customer_data.target