RSC: Extract the fetchRSC function

[Tobbe]

I gave up on splitting (see #10563)

But I did want to write down some of the findings from that experiment, and also saw an opportunity to at least to some refactoring.

The main findings were/are these:

    // React will hold on to `callServer` and use that when it detects a
    // server action is invoked (like `action={onSubmit}` in a <form>
    // element). So for now at least we need to send it with every RSC
    // request, so React knows what `callServer` method to use for server
    // actions inside the RSC.

      // I'm not sure this recursive use of `options` is needed. I briefly
      // tried without it, and things seemed to work. But keeping it for
      // now, until we learn more.
      const data = createFromFetch(response, options)