IDForwarding: Add auth hook to generate id token (#75555)

* AuthN: Move identity struct to its own file

* IDForwarding: Add IDToken property to usr and identity structs and add GetIDToken to requester interface

* Inject IDService into background services

* IDForwarding: Register post auth hook when feature toggle is enabled

pkg/services/auth/idimpl/service_test.go

@@ -0,0 +1,41 @@
+package idimpl
+
+import (
+	"testing"
+
+	"github.com/grafana/grafana/pkg/services/authn"
+	"github.com/grafana/grafana/pkg/services/authn/authntest"
+	"github.com/grafana/grafana/pkg/services/featuremgmt"
+	"github.com/grafana/grafana/pkg/setting"
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_ProvideService(t *testing.T) {
+	t.Run("should register post auth hook when feature flag is enabled", func(t *testing.T) {
+		features := featuremgmt.WithFeatures(featuremgmt.FlagIdForwarding)
+
+		var hookRegistered bool
+		authnService := &authntest.MockService{
+			RegisterPostAuthHookFunc: func(_ authn.PostAuthHookFn, _ uint) {
+				hookRegistered = true
+			},
+		}
+
+		_ = ProvideService(setting.NewCfg(), nil, nil, features, authnService)
+		assert.True(t, hookRegistered)
+	})
+
+	t.Run("should not register post auth hook when feature flag is disabled", func(t *testing.T) {
+		features := featuremgmt.WithFeatures()
+
+		var hookRegistered bool
+		authnService := &authntest.MockService{
+			RegisterPostAuthHookFunc: func(_ authn.PostAuthHookFn, _ uint) {
+				hookRegistered = true
+			},
+		}
+
+		_ = ProvideService(setting.NewCfg(), nil, nil, features, authnService)
+		assert.False(t, hookRegistered)
+	})
+}