Authn: Add function to resolve identity from org and namespace id (#84555)

* Add function to get the namespaced id

* Add function to resolve an identity through authn.Service from org and namespace id

* Switch to resolve identity for re-authenticate in another org

pkg/services/authn/authnimpl/service.go

@@ -391,6 +391,20 @@ Default:
 	return redirect, nil
 }
 
+func (s *Service) ResolveIdentity(ctx context.Context, orgID int64, namespaceID string) (*authn.Identity, error) {
+	r := &authn.Request{}
+	r.OrgID = orgID
+	// hack to not update last seen
+	r.SetMeta(authn.MetaKeyIsLogin, "true")
+
+	identity, err := s.authenticate(ctx, clients.ProvideIdentity(namespaceID), r)
+	if err != nil {
+		return nil, err
+	}
+
+	return identity, nil
+}
+
 func (s *Service) RegisterClient(c authn.Client) {
 	s.clients[c.Name()] = c
 	if cac, ok := c.(authn.ContextAwareClient); ok {