Support the `job_events` field in Slack Notification integration service

The GitLab API supports a job_events field in the Slack Notification Service Integration which is not yet available in go-gitlab.

Would be good to implement this. Also the ConfidentialNoteChannel is currently not supported. I noticed the comment in the source code here, however, if it's "only" ignored by the upstream GitLab API I think you should implement it nevertheless. It's a bug in upstream and actually documented as it should be available.