Adds control logic to rollout strategies

This makes gitlab-experiment complete in terms of being able to manage its own rollout of an experiment -- including to the control population. With this logic the library is functional without flipper, unleash, or other systems like those.

While those systems have useful tooling, this change allows us to use a poll (asking flipper, unleash, amplitude, etc.) if the feature is enabled model (current world), AND a push model, where a flipper cache adapter might push data to gitlab-experiment.

Ultimately this addition gives us a more flexible approach long term, and makes the library fully functional without needing to implement any other dependencies or your own rollout logic. It doesn't attempt to provide any tooling for that however.