feature flag subscriptions

[mfulton26]

Feature request

Is your feature request related to a problem?

I'd like my web app to always be using the latest feature flag values but not need to poll at some interval.

Describe the solution you'd like

Expose a way to subscribe to feature flags and have changes pushed by using server-sent events (SSE) text/event-stream or a WebSocket API (bi-directional communications isn't needed here though and HTTPS is my preference, so I feel like SSE is a better solution).

Describe alternatives you've considered

Poll from the app and/or set-up server-side local evaluation to create an event-stream subscription myself.

Additional context

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]