support for async factory?

[trim21]

for example:

import aiomysql
import fastapi


async def create_app():
    app = fastapi.FastAPI()

    mysql_pool = await aiomysql.create_pool(autocommit=True, minsize=4, maxsize=20)

    @app.get("/test")
    async def test():
        async with mysql_pool.acquire() as conn:
            async with conn.cursor() as cur:
                await cur.execute("...",
                                  (8,))

    return app