Authn: Always set namespace (#96230)

* Rename from AllowedKubernetesNamespace to Namespace

* Use a sync hook to always set namespace for Identity.

* format

* Don't set uid when authenticating as user

pkg/services/authn/authnimpl/sync/namespace.go

@@ -0,0 +1,28 @@
+package sync
+
+import (
+	"context"
+
+	"github.com/grafana/grafana/pkg/services/apiserver/endpoints/request"
+	"github.com/grafana/grafana/pkg/services/authn"
+	"github.com/grafana/grafana/pkg/setting"
+)
+
+func ProvideNamespaceSync(cfg *setting.Cfg) *NamespaceSync {
+	return &NamespaceSync{
+		mapper: request.GetNamespaceMapper(cfg),
+	}
+}
+
+type NamespaceSync struct {
+	mapper request.NamespaceMapper
+}
+
+func (s *NamespaceSync) SyncNamespace(ctx context.Context, id *authn.Identity, _ *authn.Request) error {
+	if id.Namespace != "" {
+		return nil
+	}
+
+	id.Namespace = s.mapper(id.OrgID)
+	return nil
+}