[Flight] Add getCacheForType() to the dispatcher (#20315)

* Remove react/unstable_cache

We're probably going to make it available via the dispatcher. Let's remove this for now.

* Add readContext() to the dispatcher

On the server, it will be per-request.

On the client, there will be some way to shadow it.

For now, I provide it on the server, and throw on the client.

* Use readContext() from react-fetch

This makes it work on the server (but not on the client until we implement it there.)

Updated the test to use Server Components. Now it passes.

* Fixture: Add fetch from a Server Component

* readCache -> getCacheForType<T>

* Add React.unstable_getCacheForType

* Add a feature flag

* Fix Flow

* Add react-suspense-test-utils and port tests

* Remove extra Map lookup

* Unroll async/await because build system

* Add some error coverage and retry

* Add unstable_getCacheForType to Flight entry

scripts/error-codes/codes.json

@@ -367,5 +367,7 @@
   "376": "Only global symbols received from Symbol.for(...) can be passed to client components. The symbol Symbol.for(%s) cannot be found among global symbols. Remove %s from this object, or avoid the entire object: %s",
   "377": "BigInt (%s) is not yet supported in client component props. Remove %s from this object or use a plain number instead: %s",
   "378": "Type %s is not supported in client component props. Remove %s from this object, or avoid the entire object: %s",
-  "379": "Refs cannot be used in server components, nor passed to client components."
+  "379": "Refs cannot be used in server components, nor passed to client components.",
+  "380": "Reading the cache is only supported while rendering.",
+  "381": "This feature is not supported by ReactSuspenseTestUtils."
 }