Add a config to experiments & rollouts to allow dependent experiments to
be flagged.
We're getting past the point where it's possible to reason about which
experiments need to be turned off if we disable some other experiment,
and so this provides some additional rollout safety.
Can be specified in both experiments and rollouts: experiments.yaml
makes the most sense and we should default to it, but rollouts.yaml lets
us put dependencies between internal & external dependencies internally
and that's gonna be a little useful.
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Co-authored-by: ctiller <ctiller@users.noreply.github.com>
Craig Tiller