Add created_at to the Billable Members API
Problem to solve
Gitlab.com customers have no way of understanding when a given user account was provisioned.
created_at field to the BillableMember entity and return as part of our billable members API resource.
Who will use this feature? If known, include any of the following: types of users (e.g. Developer), personas, or specific company roles (e.g. Release Manager). It's okay to write "Unknown" and fill this field in later.
Personas are described at https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/