CE backport for changes in EE MR 14017

What does this MR do?

CE backport for changes in EE MR https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14017 (Set static gitattributes for design repositories)

This backports to CE changes to allow the EE model DesignManagement::Repository to override the #attributes_at method to provide its own git attributes.

The #attributes_at method was freely available, as it's never called by anything in the app. It looks like the code that called it got refactored out of existence in ca66a04f. It was still being called in a spec which I've left because with the change in Lfs::FileTransformer in fact is now again the perfect test!

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team