Extract No-UI package just for compatibility

[karak]

Xamarin.Auth Issue

Feature description

Extract the part of secure storage from others, including web authentication with UI, into one dependent package as like "Xamarin.Auth.AccountStore".

Background

Many developers as I have migrated to Xamarin.Essentials and holds only AccountStore for compatibility.

Now, this remaining Xamarin.Auth causes dependency on legacy Android.Support packages on Android (see: #433) and would cause rejection by AppStore (See: #440 (comment)) on iOS, both of which are unnecessary loads.