Blazor Identity template's RevalidatingAuthenticationStateProvider should implement IHostEnvironmentAuthenticationStateProvider

[SteveSandersonMS]

To address issues with Windows authentication, we fixed #12051 via PR #12421. This provides a mechanism by which the host environment can notify the active authentication state provider that the current user principal has changed.

While this was primarily for Windows authentication, other authentication mechanisms can make use of it too. It would make sense for the "local authentication" (identity) template to also hook into this mechanism, but currently it doesn't.

To fix this, we should make RevalidatingAuthenticationStateProvider<TUser> in the project template implement IHostEnvironmentAuthenticationStateProvider. This requires a few changes to the revalidation logic to make it understand that the current user principal can change.

[SteveSandersonMS]

To be super clear, once we've made this change, we will not be using SignInManager inside the custom authentication state provider. That's broken on Azure SignalR service anyway.