feat(customers): Add or update customer currency via GraphQL

[vincent-pochet]

Context

Currently, we cannot apply a coupon, an add-on or even prepaid-credits if a customer has no active subscription. This problem happen because customer object does not hold the currency directly, but via the currency of the plan of it’s first active subscription.

It’s a problem when a plan is invoiced in-advance because it can then take up to 1 year to apply the mentioned objects on the first generated invoice.

Description

This PR allows front end to assign customer currency:

• At customer creation time via the createCustomer mutation
• At customer edition via the updateCustomer mutation

It also exposes this currency in the Customer type

In the update scenario, if the customer is attached to an active subscription, an add_on, a coupon or a wallet, the action will raise an unprocessable_entity error, with the error code currencies_does_not_match attached to the currency field