Update Direct Transfer to fetch details of banned and blocked users

Direct Transfer retrieves the source user's name and username using the GraphQL Query.users. However, due to https://gitlab.com/gitlab-org/gitlab/-/issues/346086, this query no longer returns details of users who are banned or blocked. As a result, the import source users don't get updated with the information, and placeholder users keep using a generic name. This makes it difficult for group owners to distinguish between users when reassigning.

Proposed solution

Although the GraphQL Query.users query excludes banned and blocked users, it is still possible to fetch details of individual users. To address this limitation, we should implement a fallback mechanism that retrieves individual user details when the batch operation fails to return information for a specific user. This ensures that all relevant user data is fetched, even for users excluded from the batch query.

This is step 1. of &16106

Documentation

Documentation needed here for users on how to proceed, when they come across placeholder users for users banned on source instance.