Allow passing FastAPI directly to init

[bjchambers]

This is useful in cases where the application is created in a factory or otherwise doesn't have a string directly tied to the app.

[s3rius]

Agree, sounds useful.

[lokhman]

So can this be merged?

I find it extremely useful, as in my case for using a path to the FastAPI app, I had to deal with circular dependencies.

[s3rius]

This request is closed in favor of #15. Because the mypy was not fixed for all this time.