Add missing public_email field to CreateUserOptions

The CreateUserOptions struct does not fully match the supported attributes documented in the GitLab Users API.

At the moment, it is missing several supported fields. However, a Merge Request in the Terraform provider is currently blocked (gitlab-org/terraform-provider-gitlab!2942)

because it requires support for the public_email attribute when creating users via the client API.

To unblock that work, this change adds support for:

public_email

Other missing attributes (e.g. pronouns, commit_email, auditor, etc.) will be addressed in a separate, follow-up Merge Request to keep this change focused and minimal.