Support for Confidential Note Webhook

[MiguelAngelLV]

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The webhook listener don't support Confidential Note:

org.gitlab4j.api.GitLabApiException: Unsupported X-Gitlab-Event, event Name=Confidential Note Hook

[jmini]

The confidential note hook is barely documented on: https://docs.gitlab.com/ee/user/project/integrations/webhook_events.html#comment-events

It seems that this would be an event similar to NoteEvent with a different X_GITLAB_EVENT value.

Maybe instead of duplicating NoteEvent as ConfidentialNoteEvent a common class between those two can be created. --> AbstractNoteEvent

You can open a PR with those changes.