Allow the feature flags to be enabled/disabled with more granularity

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2290

What does this MR do?

Allow the feature flags to be enabled/disabled with more granularity

This allows to enable/disable a feature flag for a given user, or a
given Flipper group (must be declared statically in the `flipper.rb`
initializer beforehand).

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

Regarding the milestone in which we want this to go, this doesn't technically fix a regression, but this will allow to re-enable the performance bar, which was supposed to land with %9.3...

Why was this MR needed?

To be able to enable the performance bar for a given set of people only.

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?

Closes #34078 (closed)