Support for SSE

[thomasp85]

Adding support for Server Sent events would be a nicety to have. I can imagine a websocket like API that only allows sending.

I'm aware that SSE is pretty limited in HTTP/1.1 but people have requested it nonetheless for plumber

[jcheng5]

This would be very nice. There's currently no streaming of responses in httpuv, period.

[jrosell]

In Python, as I understand, it seems like starlette also doesn't have SSE suport, so FastAPI uses a library that implements SSE over starlette.