Allow access to the underlying UserManager instance

[ChillyBots]

Thanks for the great work!

Currently, you don't surface the entire UserManager object which has access to callbacks and methods unavailable from anywhere else such as:

signoutRedirectCallback
signoutPopupCallback
querySessionStatus

Ideally, you could just surface the UserManager alongside the User instance

[pamapa]

We are planing to allow to inject an UserManager instance, that would help you too. See this comment.

[pamapa]

The xCallback functions are handled within this library and are one of the main purpose of this library. There is no need to call them differently. The querySessionStatus function is in fact accessible already.

[rahul-sharma-uipath]

+1 on this - we have a similar case where we want to get a static copy of the userManager in order to be able to use the access_token from static functions

[pamapa]

I added a separate issue for passing an UserManager into: #490