Add audit event on webhook creation

Problem

Audit Events are a tool for GitLab owners and administrators to view records of important actions performed across the application.

At the moment GitLab owners and administrators don't have insights into webhook creation.

Proposed solution

Add audit event for webhook creation, so that GitLab owners and administrators can have insight into who and when created webhooks.

Follow Audit event development guide: https://docs.gitlab.com/ee/development/audit_event_guide/

Customer Impact

• Premium, 2,240 seats
• Premium, 62 seats
• Premium, 250 seats

Prioritization Dashboard Score: 383,928 - currently the 12 highest issue under our group, which I would rank as high impact