Chatops control for percentage of actors rollout

Add a chatops interface for Flipper percentage of actors rollout

• The chatops command modifier --actors sets a "percentage of actors" rollout instead of the default "percentage of time" rollout

/chatops run feature set orange 23 --actors

• Gitlab feature flag API has a % of actors rollout gitlab-org/gitlab!29698 (diffs)

• This is the documentation update to "feature controls" gitlab-org/gitlab!32311 (merged)

• Main issue #78 (closed)