Add a chatops interface for Flipper 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.

https://gitlab.com/gitlab-com/chatops/-/blob/master/lib/chatops/gitlab/feature.rb

Investigation

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

Solution

Add an interface to chatops to enable percentage of actors rollout

[14] pry(main)> percentage = Feature.flipper.actors(42)
=> #<Flipper::Types::PercentageOfActors:0x00007f895738d4d8 @value=42>
[15] pry(main)> Feature.flipper[:discover_security].enable(percentage)
=> true


gitlabhq_development=# select * from feature_gates where feature_key = 'discover_security';
 id |    feature_key    |         key          | value |        created_at         |        updated_at         
----+-------------------+----------------------+-------+---------------------------+---------------------------
  5 | discover_security | percentage_of_actors | 42    | 2020-02-12 10:43:09.68394 | 2020-02-12 10:43:09.68394
(1 row)

Further