Add $required_fields array for edd_purchase_form_validate_logged_in_user function

Allows filtering of required fields on purchase form. 

I've left the first name field as required, but making the last name required is possible through the new `edd_purchase_form_required_fields` filter. Also possible is removing both the first name and last name from the validation by returning an empty array.

https://easydigitaldownloads.com/support/topic/hook-added-to-edd_purchase_form_validate_logged_in_user-function/#post-29781