Setup and run a Gitaly server for testing if GitalyClient is enabled

What does this MR do?

If Gitaly is enabled (which it will be by default after gdk reconfigure and on CI) it setups and manages a Gitaly server for testing. If so, it also makes it so that all Gitaly features are enabled while testing.

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

Open MRs will need rebase/update if their tests assume Gitaly is disabled by default.

Why was this MR needed?

As we add more features to Gitaly, it becomes more pressing to make sure the whole system works correctly with all Gitaly features enabled.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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?

https://gitlab.com/gitlab-org/gitlab-ce/issues/29066 and https://gitlab.com/gitlab-org/gitaly/issues/113