Add `force_random_password` support to `gitlab_user`

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This merge request adds a new "force_random_password" option, which allows users to set a random password for new GitLab accounts when creating users. This option conflicts with the existing "password" field, ensuring that only one of these can be used at a time. The code also includes updates to handle this new option during user creation and updates test cases to cover the new functionality.

Closes #6447 (closed)