Add ability to Read member permissions info (on the provider accounts user) via Account Mgmt API

Previous title: 'Add member permission info to the "User * (provider account)" endpoints', changed because of this.

On the provider account, member permissions for the member users can be changed with help of the PUT /admin/api/users/{user_id}.xml endpoint, using the parameters member_permission_ids[] and member_permission_service_ids[] (as described in the Knowledgebase).

However, there is no way to GET the values of the member_permission_ids and member_permission_service_ids for the user with the APIs.

The request is to add them to the User model in the API responses.