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

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

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.