Add sso.Group, context.Auth, context.APIAuth to allow auth special routes (#16086)

* Add sso.Group, context.Auth, context.APIAuth to allow auth special routes

* Remove unnecessary check

* Rename sso -> auth

* remove unused method of Auth interface

services/auth/interface.go

@@ -0,0 +1,39 @@
+// Copyright 2019 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 auth
+
+import (
+	"net/http"
+
+	"code.gitea.io/gitea/models"
+	"code.gitea.io/gitea/modules/session"
+	"code.gitea.io/gitea/modules/web/middleware"
+)
+
+// DataStore represents a data store
+type DataStore middleware.DataStore
+
+// SessionStore represents a session store
+type SessionStore session.Store
+
+// Auth represents an authentication method (plugin) for HTTP requests.
+type Auth interface {
+	Name() string
+
+	// Init should be called exactly once before using any of the other methods,
+	// in order to allow the plugin to allocate necessary resources
+	Init() error
+
+	// Free should be called exactly once before application closes, in order to
+	// give chance to the plugin to free any allocated resources
+	Free() error
+
+	// Verify tries to verify the authentication data contained in the request.
+	// If verification is successful returns either an existing user object (with id > 0)
+	// or a new user object (with id = 0) populated with the information that was found
+	// in the authentication data (username or email).
+	// Returns nil if verification fails.
+	Verify(http *http.Request, w http.ResponseWriter, store DataStore, sess SessionStore) *models.User
+}