[Fix] IAM: Add XuserType and XuserID to users.CreateOpts

[TobiWo]

What this PR does / why we need it

Add XuserType and XuserID fields to CreateOpts in openstack/identity/v3.0/users/CreateUser.go.

The OTC IAM API (POST /v3.0/OS-USER/users) accepts xuser_type and xuser_id in the request body. Both fields are already present in the User response struct and in UpdateAdminOpts, but missing from CreateOpts. This forces callers to issue a second ModifyUserAdmin call after creation to set the external identity fields, which is non-atomic.

With this change, callers can set XuserType and XuserID in a single create call.

Which issue this PR fixes

fixes #937

Special notes for your reviewer

• Both fields use omitempty so the request body is unchanged for existing
  callers that do not set these fields -> no breaking change.
• Field names and json tags match the existing convention used in
  UpdateAdminOpts (xuser_type, xuser_id).

Note

A follow-up PR will be created for the terraform-provider itself so that resource opentelekomcloud_identity_user_v3 will make use of the updated struct.

[TobiWo]

Hi @anton-sidelnikov , this PR needs review.