Enable use of scoped feature flags

Gitaly cannot currently use project-, group- or user-scoped feature flags. This only makes it possible to do global percentage-based rollouts, which isn't great and restricts our ability to test feature flags in specific circumstances. We should fix this limitation -- potentially, this needs to be done in Rails which has all the context available to actually decide whether a feature flag should be enabled or not, which would then inject the computed value into the RPC context.