feat: allow configuration of preferred username claim (#47)

SSO-Auth/Api/SSOController.cs

@@ -104,7 +104,7 @@ public class SSOController : ControllerBase
 
             foreach (var claim in result.User.Claims)
             {
-                if (claim.Type == "preferred_username")
+                if (claim.Type == (config.DefaultUsernameClaim ?? "preferred_username"))
                 {
                     StateManager[state].Username = claim.Value;
                     if (config.Roles.Length == 0)
@@ -186,7 +186,7 @@ public class SSOController : ControllerBase
                 }
             }
 
-            // If the provider doesn't support preferred_username, then use sub
+            // If the provider doesn't support the preferred username claim, then use the sub claim
             if (!StateManager[state].Valid)
             {
                 foreach (var claim in result.User.Claims)

SSO-Auth/Config/PluginConfiguration.cs

@@ -217,6 +217,11 @@ public class OidConfig
         }
         set => _canonicalLinks = value;
     }
+
+    /// <summary>
+    /// Gets or sets the default username claim when creating new accounts.
+    /// </summary>
+    public string DefaultUsernameClaim { get; set; }
 }
 
 /// <summary>