Provide webhook triggering user details with gitlabUser_* passthru

### What feature do you want to see added?

The [modern GitLab webhook events](https://docs.gitlab.com/ee/user/project/integrations/webhook_events.html) provide a `user_name`, `user_username`, and `user_email` field the identify the user who triggered the webhook event.

Currently the only user details exposed in this gitlab plugin are those of the author of the last commit (because in older webhook versions that was all that was available).

It would be useful if the event-triggering-user was available for use in the Jenkins pipelines.

Request viable for GSoC, as loosely discussed in https://github.com/jenkinsci/gitlab-plugin/issues/1005

### Upstream changes

_No response_