The customer endpoint now serves 2-letter ISO codes for countries and
accepts these for updates. It also serves and accepts region codes
(abbreviations) like VIC for Victoria.
Updates treat these fields as case-insensitive and either code or name
have to be present.
This commit also updates the Swagger documentation.
It's another read-only attribute.
Please note that JSON:API specifies a format of `date-time` which we
don't adhere to because it uses a `Z` in front of the timezone offset
which doesn't seem to be included in the default json serialisation.
So I didn't add the format and left it as simple `string`. Problem?
