Keep track of requested user ids on Gitlab client

[lampajr]

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