[Serve] Expose FastAPI docs path

[zcin]

Description

Serve integrates with FastAPI. It would be helpful if the OpenAPI docs path can be exposed / queried.

Use case

app = FastAPI(docs_url="/my_docs")

@serve.deployment
@serve.ingress(app)
class MyFastAPIDeployment:
    ...

client = serve.start()
handle = serve.run(MyFastAPIDeployment.bind(), name="app1", route_prefix="/app1")

In a separate client:

resp = requests.get("/api/serve/applications")
docs_path = extract_fastapi_docs_path(resp)
# Do something, like render openapi docs...

[zcin]

Closed with #32863.