Add ASGI gateway

[oeway]

This PR enables making ASGI apps with fastapi etc. in Pyodide. Flask is also possible with a converter.

For example, you can make a web server with ImJoy plugin:

from imjoy import api
from fastapi import FastAPI

def create_fastapi_app():
    app = FastAPI()

    @app.get("/")
    async def root():
        return {"message": "Hello World"}

    return app

async def setup():
    # Registering fastapi app
    fastapi_app = create_fastapi_app()
    async def serve_fastapi(args):
        await fastapi_app(args["scope"], args["receive"], args["send"])
    await api.register_service({
        "name": "hello-fastapi",
        "type": "ASGI",
        "serve": serve_fastapi,
        "visibility": "public"
    })

api.export({"setup": setup})

Assuming the plugin is connected in a workspace called public, the above will result in a web service served under: https://127.0.0.1:9527/public/app/hello-fastapi/.