Provide the capability to exclude specific attributes from a webhook payload
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### Proposal
- A customer raised a support ticket to enquire how to exclude a field from a webhook payload ([Customer ticket, internal-only](https://gitlab.zendesk.com/agent/tickets/616492))
- This is technically possible via [custom webhook templates](https://docs.gitlab.com/user/project/integrations/webhooks/#custom-webhook-template), _however_ it would mean reconstructing the rest of the usual webhook payload structure
- This would be tedious as there are many attributes on a webhook payload, [for example a pipeline event webhook payload has 200+ attributes](https://docs.gitlab.com/user/project/integrations/webhook_events/#pipeline-events)
- To make excluding specific attributes more straightforward and practical the customer linked above requested that we consider implementing a function to exclude specific attributes inside GitLab - this function could appear in the interface in the same area as the custom webhook template fields
issue