Allow self-managing session refetch

[rafde]

Description 📓

The way I handle session updates is different from how SessionProvider does it. I have my own SessionProvider that would call a debounced getSession that fires when there's keydowns, mouse moves, or scrolls, etc., which would update the session state if there were any updates to the fetched data.
Now I have no choice but to use what is provided by this library since it creates an internal getSession only through SessionProvider, and it's needed in order to use signOut.

I would be fine with this but using the library version is causing me issues that I didn't have when I handled it myself.

What I think would help is if
https://github.com/nextauthjs/next-auth/blob/v4.2.1/src/react/index.tsx#L320-L359
was exposed through https://github.com/nextauthjs/next-auth/blob/v4.2.1/src/react/index.tsx#L410-L415
as something like

      fetchSession: () => __NEXTAUTH._getSession(),

since it does all the event broadcasting.

How to reproduce ☕️

no example.

Contributing 🙌🏽

Yes, I am willing to help implement this feature in a PR