Auth: Introduce pre-logout hooks + add GCOM LogoutHook (#88475)

* Introduce preLogoutHooks in authn service

* Add gcom_logout_hook

* Config the api token from the Grafana config file

* Simplify

* Add tests for logout hook

* Clean up

* Update

* Address PR comment

* Fix

pkg/services/auth/gcomsso/gcom_logout_hook_test.go

@@ -0,0 +1,64 @@
+package gcomsso
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/grafana/grafana/pkg/models/usertoken"
+	"github.com/grafana/grafana/pkg/services/user"
+	"github.com/grafana/grafana/pkg/setting"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGComSSOService_LogoutHook(t *testing.T) {
+	cfg := &setting.Cfg{
+		GrafanaComURL:         "http://example.com",
+		GrafanaComSSOAPIToken: "sso-api-token",
+	}
+
+	s := ProvideGComSSOService(cfg)
+
+	t.Run("Successfully logs out from grafana.com", func(t *testing.T) {
+		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			require.Equal(t, http.MethodPost, r.Method)
+			require.Equal(t, "/api/logout/grafana/sso", r.URL.Path)
+
+			require.Equal(t, "application/json", r.Header.Get("Content-Type"))
+			require.Equal(t, "Bearer "+cfg.GrafanaComSSOAPIToken, r.Header.Get("Authorization"))
+
+			w.WriteHeader(http.StatusNoContent)
+		}))
+		defer server.Close()
+
+		cfg.GrafanaComURL = server.URL
+		user := &user.SignedInUser{
+			IDToken: "id-token",
+		}
+		sessionToken := &usertoken.UserToken{
+			Id: 123,
+		}
+
+		err := s.LogoutHook(context.Background(), user, sessionToken)
+		require.NoError(t, err)
+	})
+
+	t.Run("Fails to log out from grafana.com", func(t *testing.T) {
+		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusInternalServerError)
+		}))
+		defer server.Close()
+
+		cfg.GrafanaComURL = server.URL
+		user := &user.SignedInUser{
+			IDToken: "id-token",
+		}
+		sessionToken := &usertoken.UserToken{
+			Id: 123,
+		}
+
+		err := s.LogoutHook(context.Background(), user, sessionToken)
+		require.Error(t, err)
+	})
+}