Refactor system setting (#27000) (#27452)

Backport #27000 by @wxiaoguang

This PR reduces the complexity of the system setting system.

It only needs one line to introduce a new option, and the option can be
used anywhere out-of-box.

It is still high-performant (and more performant) because the config
values are cached in the config system.


![image](https://github.com/go-gitea/gitea/assets/2114189/f8cdd743-1145-41ab-9f8f-3996aa97d440)

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/setting/config/getter.go

@@ -0,0 +1,49 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package config
+
+import (
+	"context"
+	"sync"
+)
+
+var getterMu sync.RWMutex
+
+type CfgSecKeyGetter interface {
+	GetValue(sec, key string) (v string, has bool)
+}
+
+var cfgSecKeyGetterInternal CfgSecKeyGetter
+
+func SetCfgSecKeyGetter(p CfgSecKeyGetter) {
+	getterMu.Lock()
+	cfgSecKeyGetterInternal = p
+	getterMu.Unlock()
+}
+
+func GetCfgSecKeyGetter() CfgSecKeyGetter {
+	getterMu.RLock()
+	defer getterMu.RUnlock()
+	return cfgSecKeyGetterInternal
+}
+
+type DynKeyGetter interface {
+	GetValue(ctx context.Context, key string) (v string, has bool)
+	GetRevision(ctx context.Context) int
+	InvalidateCache()
+}
+
+var dynKeyGetterInternal DynKeyGetter
+
+func SetDynGetter(p DynKeyGetter) {
+	getterMu.Lock()
+	dynKeyGetterInternal = p
+	getterMu.Unlock()
+}
+
+func GetDynGetter() DynKeyGetter {
+	getterMu.RLock()
+	defer getterMu.RUnlock()
+	return dynKeyGetterInternal
+}