Add customer_id to Spree::User

[nirebu]

In order to create a KObject belonging to a customer to make it appear on their timeline we need to hit an endpoint such as

"/customers/#{kustomer_customer_UUID}/klasses/#{kobject_klass_name}"

to get that ID we need first to query Kustomer for it. We can get it by querying for a Spree::User id, which usually is used as externalId on Kustomer like so:

"/customers/externalId=#{Spree::User.id"

To reduce the amount of requests to Kustomer could be a good idea to store this ID in the user table.

[stale]

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