Add `createGitlabTokenValidator(config)` which can be used
to create a validator used at an ingress for topic `gitlab`.
On top, there is a new `gitlabWebhookEventsModule` for the new backend plugin API
which auto-registers the `HttpPostIngress` for topic `gitlab` incl. the validator.
Relates-to: PR #13931
Signed-off-by: Patrick Jungermann <Patrick.Jungermann@gmail.com>