Add X-Gitlab-Webhook-UUID header to web hooks

What does this MR do and why?

This adds a unique identification header for every webhook event that comes from GitLab.

This can be used by webhook receivers to correlate logs that handled the webhook and aid in the troubleshooting and debugging of webhooks.

#230830 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #230830 (closed)