Implement a web hook create service

What does this MR do and why?

This change moves the creation logic for web hooks into a separate service class.

The service class is used for both the UI and REST API entrypoints in GitLab.

This is needed for an upcoming change to implement audit events for web hook creation #8068 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

N/A as this is purely a code change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

The functionality should remain the same - to verify this you can create a project, group and system hook.