Support retrieval of disk statistics from Gitaly

What does this MR do?

This MR adds a rudimentary client implementation for the new gitaly disk_statistics grpc, along with appropriate unit tests.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

No functionality implementation; client method implementations and tests only; risks are very little to none; quality or performance impact is very little to none; additional unit tests covering client methods are included; no cross-browser testing required.

• 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