fix: attach timestamp to impression events

[tyiuhc]

Summary

This update will attach timestamps to $impression events when they are incurred.

This is necessary for consent-aware exposure tracking, which keeps $impression events in memory when consent status is PENDING. When status is updated to GRANTED, the timestamp is required to properly track when the event was incurred, and not when the event was processed by the analytics SDK.

Relevant analytics-browser PR.

Checklist

• Does your PR title have the correct title format?
• Does your PR have a breaking change?: No

[promptless]

📝 Documentation updates detected!

New suggestion: Document Exposure object fields including timestamp for custom exposure tracking

[Mercy811]

I've limited knowledge about experiment SDK but it looks good to me as Exposure is just event properties and it aligns with the other pr in analytics SDK.