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](f8cdd743-1145-41ab-9f8f-3996aa97d440)

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

modules/setting/config.go

@@ -0,0 +1,55 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package setting
+
+import (
+	"sync"
+
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/setting/config"
+)
+
+type PictureStruct struct {
+	DisableGravatar       *config.Value[bool]
+	EnableFederatedAvatar *config.Value[bool]
+}
+
+type ConfigStruct struct {
+	Picture *PictureStruct
+}
+
+var (
+	defaultConfig     *ConfigStruct
+	defaultConfigOnce sync.Once
+)
+
+func initDefaultConfig() {
+	config.SetCfgSecKeyGetter(&cfgSecKeyGetter{})
+	defaultConfig = &ConfigStruct{
+		Picture: &PictureStruct{
+			DisableGravatar:       config.Bool(false, config.CfgSecKey{Sec: "picture", Key: "DISABLE_GRAVATAR"}, "picture.disable_gravatar"),
+			EnableFederatedAvatar: config.Bool(false, config.CfgSecKey{Sec: "picture", Key: "ENABLE_FEDERATED_AVATAR"}, "picture.enable_federated_avatar"),
+		},
+	}
+}
+
+func Config() *ConfigStruct {
+	defaultConfigOnce.Do(initDefaultConfig)
+	return defaultConfig
+}
+
+type cfgSecKeyGetter struct{}
+
+func (c cfgSecKeyGetter) GetValue(sec, key string) (v string, has bool) {
+	cfgSec, err := CfgProvider.GetSection(sec)
+	if err != nil {
+		log.Error("Unable to get config section: %q", sec)
+		return "", false
+	}
+	cfgKey := ConfigSectionKey(cfgSec, key)
+	if cfgKey == nil {
+		return "", false
+	}
+	return cfgKey.Value(), true
+}