Add Default External Reference and Default Customer Order to Location endpoint

[kbrevik]

We have a third party system that keeps track of these fields.
It would be nice if these fields could be available on the Locations endpoint (f.ex. via. /api/v3/Customers/{customerId}/locations).
Then we could inherit these values if they are set when we sync customer data.

[MagnusJ]

Also want the ability to fetch these values!