Add system setting table with cache and also add cache supports for user setting (#18058)

modules/system/appstate.go

@@ -0,0 +1,25 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package system
+
+// StateStore is the interface to get/set app state items
+type StateStore interface {
+	Get(item StateItem) error
+	Set(item StateItem) error
+}
+
+// StateItem provides the name for a state item. the name will be used to generate filenames, etc
+type StateItem interface {
+	Name() string
+}
+
+// AppState contains the state items for the app
+var AppState StateStore
+
+// Init initialize AppState interface
+func Init() error {
+	AppState = &DBStore{}
+	return nil
+}