Add "deny disk access" Gitaly feature (tripswitch)

What does this MR do?

This change adds a Gitaly feature that allows us to simulate gitlab-rails having no direct access to Git repositories anymore. If the feature is enabled, a disk path lookup for a repository will raise an exception.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

This helps us hunt for production code that still uses direct disk access to Git repositories.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes gitaly#1210 (closed)