fix: #143 Add dedicated mutation to assign vatRate to customer

[vincent-pochet]

Fixes: #143

Add a new updateCustomerVatRate mutation to allow front to change the customer vat_rate without providing customer mandatory fields like customerId or name

This changes is required because the front cannot ensure name and customer id are in sync with the back when updating the vat_rate (the view is not the same) and so there is a risk of erasing/overriding changes on customer.