Server Components HMR Cache (#67527)
This adds support for caching `fetch` responses in server components
across HMR refresh requests. The two main benefits are faster responses
for those requests, and reduced costs for billed API calls during local
development.
**Implementation notes:**
- The feature is guarded by the new experimental flag
`serverComponentsHmrCache`.
- The server components HMR cache is intentionally independent from the
incremental cache.
- Fetched responses are written to the cache after every original fetch
call, regardless of the cache settings (specifically including
`no-store`).
- Cached responses are read from the cache only for HMR refresh
requests, potentially also short-cutting the incremental cache.
- The HMR refresh requests are marked by the client with the newly
introduced `Next-HMR-Refresh` header.
- I shied away from further extending `renderOpts`. The alternative of
adding another parameter to `renderToHTMLOrFlight` might not necessarily
be better though.
- This includes a refactoring to steer away from the "fast refresh"
wording, since this is a separate (but related) [React
feature](https://github.com/facebook/react/issues/16604#issuecomment-528663101)
(also build on top of HMR).
x-ref: #48481
