provide revalidateReason to getStaticProps (#64258)
Provides a `revalidateReason` argument to `getStaticProps` ("stale" |
"on-demand" | "build").
- Build indicates it was run at build time
- On-demand indicates it was run as a side effect of [on-demand
revalidation](https://nextjs.org/docs/pages/building-your-application/data-fetching/incremental-static-regeneration#on-demand-revalidation)
- Stale indicates the resource was considered stale (either due to being
in dev mode, or an expired revalidate period)
This will allow changing behavior based on the context in which it's
called.
Closes NEXT-1900
