Introduce webhook rate limiting setting through API

Proposal

Introduce webhook rate limiting setting through API, so that self-managed and Dedicated customers can manage this limit without entering into Rails console.

• Documentation to update via Rails console: https://docs.gitlab.com/administration/instance_limits/#webhook-rate-limit.

It was initially proposed to introduce the webhook rate limiting setting through the application setting in the UI, however @rodrigo.tomonari noted the following:

The easiest solution is to enable configuration support via the API.

Webhook rate limiting isn’t an application setting; instead, it's a Plan setting.

We would need a UI that allows users to configure settings for each plan, making the task more complex than adding a basic application setting.

May I modify the proposed solution to reference the API instead of the UI?