Add an API for Flipper percentage of actors rollout

Add an API backend for the chatops percentage_of_actors rollout,

Currently the chatops interface for feature flags only allows percentage_of_time rollout.

For experiments with feature rollout percentage_of_actors a way to rollout device-independent features based on rollout.

As part of the issue "Investigate use of Percentage of Actors Flipper Feature" #205206 (closed) it looks like percentage_of_actors could add value

• Documentation https://docs.gitlab.com/ee/development/feature_flags/controls.html
• Chatops interface MR #214676 (moved)
• Existing API https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/api/features.rb#L67