Add Cloud Connector service design doc

What does this MR do and why?

In this design document, we explore the idea of introducing a dedicated Cloud Connector service gateway.

This discussion started in and should replace #425423 (closed).

Preview: https://main-ee-132977.docs.gitlab-review.app/ee/architecture/blueprints/cloud_connector/service_gateway.html

Screenshots or screen recordings

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

Before	After

How to set up and validate locally

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

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 #425423 (closed)