Changes to Basic Auth to support external logins (#12434)

* Fixed issues with basic auth middleware to support Umbraco Cloud usecase

* Fix redirects to return url, now allows website urls

* Strip potential domain part of returnPath

* Fix default value in appsettings schema

* Reintroduce check of basic auth enabled.

* Fix wrong negation introduced in #12349

* Fixed issues with redirects

* Also check external login cookie, while authenticating backoffice

src/Umbraco.Core/Configuration/Models/BasicAuthSettings.cs

@@ -23,5 +23,18 @@ namespace Umbraco.Cms.Core.Configuration.Models
 
 
         public string[] AllowedIPs { get; set; } = Array.Empty<string>();
+        public SharedSecret SharedSecret { get; set; } = new SharedSecret();
+
+        public bool RedirectToLoginPage { get; set; } = false;
+
+    }
+
+    public class SharedSecret
+    {
+        private const string StaticHeaderName =  "X-Authentication-Shared-Secret";
+
+        [DefaultValue(StaticHeaderName)]
+        public string? HeaderName { get; set; } = StaticHeaderName;
+        public string? Value { get; set; }
     }
 }