Feature Request: Set flag distribution in bootstrap

Feature request

Is your feature request related to a problem?

The custom Fast feature flags library allows users to bootstrap flags and set distribution in the frontend, this makes it useful for use cases where users want to avoid flicker which happens when waiting for flag evaluation.

Describe the solution you'd like

A way to set flag distribution from the SDK that's similar to this:

     const assignedFeatureFlags = PFFF([
       {
         key: 'blog-post-cta',
         variants: {
           control: 0.50,
           test: 0.50,
         },
       }]);

From: https://github.com/danielbachhuber/posthog-fast-feature-flags/blob/main/src/posthog-fast-feature-flags.ts

Describe alternatives you've considered

• Use bootstrapping, but calculate the flags on the server or a view before the one where the flag is needed immediately

Additional context

From: https://posthoghelp.zendesk.com/agent/tickets/34303 (moved to PostHog: https://us.posthog.com/project/2/support/tickets/35904)

Debug info

null

Start by reviewing the referenced src/posthog-fast-feature-flags.ts implementation and the bootstrapping documentation to understand the requested frontend behavior. Clarify the target PostHog SDK entry point and interface with maintainers; done should mean the distribution can be specified from the SDK without relying on server-side flag calculation.