Keep track of requested user ids on Gitlab client

In the Gitlab client we have to perform additional request to retrieve the user ids in order to correctly setup reviewers and assignees, since it could happen that an assignee is also a reviewers (especially if you consider that backporting is generally automatically executed by CI/CD tools) we could keep track of already requested user ids and check it before doing the rest call.

From a technical point of view this could be implemented as simple key-value map where the key is the `username` and the value is the `userId`.

Start at the GitLab client and trace the requests used to resolve users for reviewers and assignees. Implement the username-to-user ID tracking described in the issue, then verify that repeated users reuse the stored ID and do not trigger another REST request.

Written by the indexing model from the issue text.