Add system service for Focalboard multi product architecture (#20574)

* add system service; add GetUsers to users service
* add feature flag

app/app.go

@@ -173,3 +173,12 @@ func (a *App) SetServer(srv *Server) {
 func (a *App) UpdateExpiredDNDStatuses() ([]*model.Status, error) {
 	return a.Srv().Store.Status().UpdateExpiredDNDStatuses()
 }
+
+// systemServiceAdapter provides a collection of system APIs for use by products.
+type systemServiceAdapter struct {
+	server *Server
+}
+
+func (ssa *systemServiceAdapter) GetDiagnosticId() string {
+	return ssa.server.TelemetryId()
+}

app/server.go

@@ -103,6 +103,7 @@ const (
 	HooksKey         ServiceKey = "hooks"
 	KVStoreKey       ServiceKey = "kvstore"
 	StoreKey         ServiceKey = "storekey"
+	SystemKey        ServiceKey = "systemkey"
 )
 
 type Server struct {
@@ -404,6 +405,7 @@ func NewServer(options ...Option) (*Server, error) {
 		CloudKey:         &cloudWrapper{cloud: s.Cloud},
 		KVStoreKey:       &kvStoreWrapper{srv: s},
 		StoreKey:         store.NewStoreServiceAdapter(s.Store),
+		SystemKey:        &systemServiceAdapter{server: s},
 	}
 
 	// Step 8: Initialize products.

app/team.go

@@ -34,11 +34,11 @@ type teamServiceWrapper struct {
 	app AppIface
 }
 
-func (w *teamServiceWrapper) GetMember(teamID, userID string) (*model.TeamMember, error) {
+func (w *teamServiceWrapper) GetMember(teamID, userID string) (*model.TeamMember, *model.AppError) {
 	return w.app.GetTeamMember(teamID, userID)
 }
 
-func (w *teamServiceWrapper) CreateMember(ctx *request.Context, teamID, userID string) (*model.TeamMember, error) {
+func (w *teamServiceWrapper) CreateMember(ctx *request.Context, teamID, userID string) (*model.TeamMember, *model.AppError) {
 	return w.app.AddTeamMember(ctx, teamID, userID)
 }