Per discussion over on #5441 it'd be good to have a globally accessible framework guarding high risk features, as well as tooling to handle the lifetime of migration from old code to new, which matches what we do for config guarding including warning/hard-fail (see #5559) and scripts to advance from warning to defaut-off to filing clean-up issues.

Plan A is to use Envoy's existing run-time, but make it a singleton to make it easy for any code in Envoy to check in on the status of the relevant features.

Eventually it'd be nice if flags could also pulled over a discovery service, as well as set via command line for those of us who are used to flag based features

This issue covers both the code changes and tooling changes as one without the other isn't particularly usable.

I'm going to stick this on the agenda for next week's community meeting, but I'd appreciate folks in the community and @envoyproxy/maintainers weighing in early and often with thoughts of how to make this useful and easy to use.