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/clients/identity.go

@@ -0,0 +1,33 @@
+package clients
+
+import (
+	"context"
+
+	"github.com/grafana/grafana/pkg/services/authn"
+)
+
+var _ authn.Client = (*IdentityClient)(nil)
+
+func ProvideIdentity(namespaceID string) *IdentityClient {
+	return &IdentityClient{namespaceID}
+}
+
+type IdentityClient struct {
+	namespaceID string
+}
+
+func (i *IdentityClient) Name() string {
+	return "identity"
+}
+
+// Authenticate implements authn.Client.
+func (i *IdentityClient) Authenticate(ctx context.Context, r *authn.Request) (*authn.Identity, error) {
+	return &authn.Identity{
+		OrgID: r.OrgID,
+		ID:    i.namespaceID,
+		ClientParams: authn.ClientParams{
+			FetchSyncedUser: true,
+			SyncPermissions: true,
+		},
+	}, nil
+}