Route path lookups through legacy_disk_path

What does this MR do?

Gain more control over how git repository paths are accessed, with the goal of banning git repository access from production code.

We do this by forcefully injecting the new GitalyClient::StorageSettings class and only making the real 'path' value accessible via the new 'legacy_disk_path' accessor. The idea is that this gives us something to grep for in the code, and a place where we can do things like conditionally raising an exception.

Although I looked hard, there is a good chance that this MR breaks something. All I had to go on was grepping for the word 'path' and the test suite.

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

Why was this MR needed?

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
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?