[Next.js] Mark Cold Start When Deployed To Vercel

### Problem Statement

Customers would like to be able to differentiate events based on whether or not they are associated with a cold-start of a severless function.

The initial request specifically mentions Next.js when deployed to vercel.

### Solution Brainstorm

Identifying whether or not the event context is from a cold-start using a tag may be very helpful.

Examples:

- alerting on % change for transaction duration only when considering a hot-start
- knowing whether an error is associated only with cold starts

cc @lforst