Allow setting fastapi root_path argument

[chiragjn]

When FastAPI is run behind a path-based proxy the swagger fails to render because it is not aware of the path the application root is on
E.g. If the app is hosted on https://example.com/vllm-svc/ then
with root_path=None it will look for openapi.json at https://example.com/openapi.json and fail with 404
with root_path="vllm-svc" it will look for openapi.json at https://example.com/vllm-svc/openapi.json which works correctly

This change only affects the swagger at /docs
See: https://fastapi.tiangolo.com/advanced/behind-a-proxy/#proxy-with-a-stripped-path-prefix

[chiragjn]

@zhuohan123 Would appreciate if you can review when you get time