IDForwarding: add "authenticatedBy" to id token (#80622)

* IDForwading: Set authenticated by for users

pkg/services/auth/idtest/mock.go

@@ -0,0 +1,18 @@
+package idtest
+
+import (
+	"context"
+
+	"github.com/grafana/grafana/pkg/services/auth"
+)
+
+type MockSigner struct {
+	SignIDTokenFn func(ctx context.Context, claims *auth.IDClaims) (string, error)
+}
+
+func (s *MockSigner) SignIDToken(ctx context.Context, claims *auth.IDClaims) (string, error) {
+	if s.SignIDTokenFn != nil {
+		return s.SignIDTokenFn(ctx, claims)
+	}
+	return "", nil
+}