Create a field for storing remote IDs

Payment gateways often need to create a remote customer for the current user.
We need a custom field that will allow us to store those IDs, and their providers (Braintree, Stripe, etc).
That field will be able to get and set IDs by provider.

That's the primary purpose of the field. Other uses may appear over time (orders?), so it makes sense for this field to live in the base Commerce module.

This field has already been developed in the payment branch.