Draft: Add state machine for better rollout tooling

This implements a base state machine that outlines the process workflow of experiment rollout as far as we currently understand it.

An experiment starts in an :off state. You can run! an experiment, which transitions it to a :running state. A :running experiment can be :paused by using pause!, or marked as :resolved by using resolve! (a variant can optionally be specified with resolve!(:variant_name)). When an experiment is :paused, it can be placed back into a :running state, or can be ended using terminate! (which marks the experiment as :resolved to the control.)

The experiment state event methods are as follows:

• run! to transition from an :off or :paused state to a :running state
• pause! to transition from a :running state to a :paused state
• resolve!([variant_name]) to transition from a :running or :paused state to a :resolved state using the specified variant (the first registered variant being the default)
• terminate! to transition from a :running or :paused state to a :resolved state using the control

Experiments in the following states will exhibit the specified behavior:

• :off immediately routes to the control and tracking events will be ignored
• :running allow the rollout strategy to assign a behavior and events will be tracked
• :paused immediately routes to the control behavior and tracking events will be ignored
• :resolved immediately routes to the resolved behavior (as cached) and tracking events will be ignored