proxyHandler should allow for `bufferOutput=false`

[jtmcdole]

I'm currently using flutter web-server with proxy + dart_frog + SSE. I've noticed that since using the proxy, my SSE handlers are starved on the client side. I suspect the proxyHandler is buffering the output, but I'm not sure.

https://github.com/flutter/flutter/blob/75cca889942c2bda1599f5ef3f46aeefbd4f7ab7/packages/flutter_tools/lib/src/isolated/proxy_middleware.dart#L43-L47