Expose API for user provided SessionProvider

[cleverchuk]

There isn't a public API for users to provide their own session manager or provider implementation. The existing API is package private. How about we expose an API for users to provide their own SessionProvider and SessionManager?

This would be useful for a use case where a user session represents a sequence of tasks that must be completed or canceled before the session can be closed or ended. The current implementation doesn't allow for that and uses arbitrary session identifier which may not represent how a user wants a session to be identified.

[bidetofevil]

Feels like this is something we should allow given the interface is public for a reason. I'll take a look

[breedx-splk]

I still feel like we're being too generous with the session abstraction and should be more opinionated about it. We can/should expose the id generator for advanced users, but the behavior I think we should be better about managing. 🤷🏻