Create a specific ServerSentEventGenerator version that matches the w…

[andredias]

This pull request creates a specific ServerSentEventGenerator version that matches the way FastAPI handles SSE since version 0.135

It doesn't break other versions.

[gazpachoking]

So, turns out you can already do this in the new FastAPI without this PR!

@app.get("/updates", response_class=DatastarResponse)
def updates():
    yield ServerSentEventGenerator.patch_elements("""<div id="myDiv"></div>""")

Should work out of the box. How do you feel about just recommending that way rather than rewriting how it works in the SDK to allow setting response_class=EventSourceResponse?

The bad news is that the datastar_response decorator is broken with the new fastapi versions. I'd like to find a way to fix that.

[gazpachoking]

I think given that DatastarResponse already works with the new fastapi generator based streaming there isn't a need to implement support for EventstreamResponse. I opened up #44 to remove the datastar_response decorator for fastapi and document the alternative.