factor out auth constants, remove TAlias is Alias is not an extensible type, improve SiteOptions cache clearing, improve principal validation, localization improvements

Oqtane.Shared/Shared/AuthenticationProviderTypes.cs

@@ -0,0 +1,6 @@
+namespace Oqtane.Shared {
+    public class AuthenticationProviderTypes {
+        public const string OpenIDConnect = "oidc";
+        public const string OAuth2 = "oauth2";
+    }
+}