dillon1000/react

[Flight Reply] Encode ReadableStream and AsyncIterables (#28893)

Same as #28847 but in the other direction.

Like other promises, this doesn't actually stream in the outgoing
direction. It buffers until the stream is done. This is mainly due to
our protocol remains compatible with Safari's lack of outgoing streams
until recently.

However, the stream chunks are encoded as separate fields and so does
support the busboy streaming on the receiving side.