included `evaluated_at` properties in `$feature_flag_called` events

[dmarticus]

:bulb: Motivation and Context

See https://github.com/PostHog/posthog/pull/41824 for context

:green_heart: How did you test it?

TODO

:pencil: Checklist

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