Add Relay Flight Build (#18242)

* Rename to clarify that it's client-only

* Rename FizzStreamer to FizzServer for consistency

* Rename react-flight to react-client/flight

For consistency with react-server. Currently this just includes flight
but it could be expanded to include the whole reconciler.

* Add Relay Flight Build

* Rename ReactServerHostConfig to ReactServerStreamConfig

This will be the config specifically for streaming purposes.
There will be other configs for other purposes.