feat: Implement customer invitation infrastructure in Authentik

- Added IInvitationSetupService and InvitationSetupService to orchestrate the setup of invitation infrastructure for customers.
- Introduced methods for creating groups, enrollment flows, invitation stages, roles, and policies in Authentik.
- Updated PostInstanceInitService to call the new invitation setup methods during post-initialization.
- Enhanced InstanceService to pass customer name during SAML configuration deployment.
- Updated App.axaml.cs to register the new IInvitationSetupService.
- Modified settings-custom.php.template to include documentation for SAML authentication configuration with group-based admin assignment.
- Added logic to exclude specific groups from being synced to Xibo during group synchronization.

OTSSignsOrchestrator.Desktop/App.axaml.cs

@@ -158,6 +158,7 @@ public class App : Application
         services.AddTransient<InstanceService>();
         services.AddTransient<IBitwardenSecretService, BitwardenSecretService>();
         services.AddTransient<IAuthentikService, AuthentikService>();
+        services.AddTransient<IInvitationSetupService, InvitationSetupService>();
         services.AddSingleton<PostInstanceInitService>();
 
         // ViewModels