Support on feature flags loaded

[nrobi144]

:bulb: Motivation and Context

Closes https://github.com/PostHog/posthog-flutter/issues/81

:green_heart: How did you test it?

:warning: Requires testing after initial feedback

:pencil: Checklist

• [x] I reviewed the submitted code.
• [ ] I added tests to verify the changes.
• [ ] I updated the docs and example project if needed.
• [x] No breaking change or entry added to the changelog.