Update storage settings to allow extra values per shard

What does this MR do?

Update storage settings to allow extra values per shard

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

No

Why was this MR needed?

As discussed in #28447 (closed), we'll need a mechanism to specify a Gitaly address for each repository storage. This will imply a change in the structure of the storage settings. We are probably not going to be able to get the required Gitaly changes into 9.0, but updating the structure of the settings now to allow for extra values per storage will prevent us from introducing breaking changes in minor versions

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Related to #28447 (closed)