Add Auditor field in ModifyUserOptions

Giannis Kepasrequested to merge
gitlab-community/gitlab-org/api/client-go:modify-user-options-auditor into main

It was discovered in gitlab-org/terraform-provider-gitlab!3163 that the ModifyUserOptions struct in users.go did not expose the Auditor field while the Users API returns it normally.

This MR adds the field as well as a test function.

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