Add placeholder user mapping to bitbucket server importer

What does this MR do and why?

Mentions https://gitlab.com/gitlab-org/gitlab/-/issues/466356

MR acceptance checklist

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Before	After

How to set up and validate locally

• Set up a Bitbucket Server instance by following these instructions.
• Set up your project and repository on your BBS instance.
  • As UserA:
    • Create UserB in http://localhost:7990/admin/users and set their password.
    • Create a project with a simple README and open an MR with a modification.
    • Add UserB to the project and assign UserB as a reviewer.
    • Make inline/standalone comments on the MR.
  • As UserB:
    • Make inline/standalone comments on the MR.
    • Approve and merge the MR.
  • Make any other contributions on your source instance that you'd like to see imported.
• Enable BBS import in your GDK and enable the relevant feature flags: bitbucket_server_user_mapping & importer_user_mapping.
• Import a project from http://localhost:7990 to the target via BitBucket Server importer.
• On the target instance, go to the target group that you just imported the project into and go to the group members page. Ensure that your source contributions are mapped as expected.