Add EventResponse to support Generator/Asynchronous Generator

[Aber]

example

@app.register
def generator_function():
    yield 1
    yield 2

@app.register
async def generator_function():
    yield 1
    yield 2

[Aber]

https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events looks like a great implementation