Move HTTP service to public for plugin use (#27284)

* Move httpservice for use in plugins

* Adapt httpservice for plugin use

* Fix lint

server/public/shared/configservice/configservice.go

@@ -0,0 +1,15 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package configservice
+
+import (
+	"github.com/mattermost/mattermost/server/public/model"
+)
+
+// An interface representing something that contains a Config, such as the app.App struct
+type ConfigService interface {
+	Config() *model.Config
+	AddConfigListener(func(old, current *model.Config)) string
+	RemoveConfigListener(string)
+}