chore(flags, activity): show $feature_flag_payload as official PostHog evt property

[havenbarnes]

Problem

Making this tweak as a follow up to the very end of this customer support ticket: https://posthoghelp.zendesk.com/agent/tickets/21126

Changes

👉 Stay up-to-date with PostHog coding conventions for a smoother review.

Does this work well for both Cloud and self-hosted?

Yes

How did you test this code?

Tested visually after triggering some events with posthog-node

[github-actions]

Size Change: +180 B (+0.02%)

Total Size: 1.16 MB

ℹ️ View Unchanged

Filename	Size	Change
frontend/dist/toolbar.js	1.16 MB	+180 B (+0.02%)

_{compressed-size-action}