[Flight Fixture] Proxy requests through the global server instead of directly (#26257)

This proxies requests through the global server instead of requesting
RSC responses from the regional server. This is a bit closer to
idiomatic, and closer to SSR.

This also wires up HMR using the Middleware technique instead of server.
This will be an important part of RSC compatibility because there will
be a `react-refresh` aspect to the integration.

This convention uses `Accept` header to branch a URL between HTML/RSC
but it could be anything really. Special headers, URLs etc. We might be
more opinionated about this in the future but now it's up to the router.

Some fixes for Node 16/17 support in the loader and fetch polyfill.