Add Design Repository model

What does this MR do and why?

This MR introduces the model Projects::DesignRepository as a representation of design repo data. This is in preparation for replication and verification by Geo's Self-service framework in #355660 (closed).

A similar approach was used to migrate Wiki Repositories to SSF in !103399 (merged)

Screenshots or screen recordings

How to set up and validate locally

Rollback a migration in gitlab_main and rerun to see no new changes in structure.sql.

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.