Allow extensions to add a credential manager more easily

In NICE DCV we recently implemented an extension that provides Single Sign-On functionality. We implemented our CredentialManager that mimics the behavior of the ones already present in the codebase (oVirt and VMware). However, in order to make it works as expected, the extension accesses some private variables or implements a function which is almost the same as the original one.

For example, you can see here that we copy-pasted entirely the AuthPrompt.reset() function just for adding our own service in the preauthenticated check. Here instead, we poke at the ScreenShield internals in order to activate it.

This merge request proposes some minor refactoring that would avoid the issues above. This is marked as draft in order to collect feedback on the proposed API.