Add feature toggles through Flipper

What does this MR do?

Add feature toggles through Flipper to allow percentages and other gate criteria

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

How does this relate to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4565 ?

Why was this MR needed?

The current process to enable/disable features in Gitaly needs improvement (See gitaly#232 (closed))

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
  • 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?

gitaly#232 (closed)