Consume the RSC stream twice in the Flight fixture (#28353)

We have an unresolved conflict where the Flight client wants to execute
inside Fizz to emit side-effects like preloads (which can be early) into
that stream. However, the FormState API requires the state to be passed
at the root, so if you're getting that through the RSC payload it's a
Catch 22.

#27314 used a hack to mutate the form state array to fill it in later,
but that doesn't actually work because it's not always an array. It's
sometimes null like if there wasn't a POST. This lead to a bunch of
hydration errors - which doesn't have the best error message for this
case neither. It probably should error with something that specifies
that it's form state.

This fixes it by teeing the stream into two streams and consuming it
with two Flight clients. One to read the form state and one to emit
side-effects and read the root.