fix: mounted apps don't eagerly match path prefix

[frostming]

Signed-off-by: Frost Ming me@frostming.com

What does this PR address?

With this change, the following will be possible:

import bentoml
from fastapi import FastAPI

app = FastAPI()


@app.get("/hello")
def hello():
    return {"Hello": "World"}


@bentoml.service
@bentoml.asgi_app(app, path="/")
class MyService:
    pass

Visiting / renders the swagger page, and visiting /hello gives the fastapi response. Previously, the app mounted at the root path / will shadow all internal endpoints