Map OIDC groups to Orgs/Teams (#21441)

Fixes #19555

Test-Instructions:
https://github.com/go-gitea/gitea/pull/21441#issuecomment-1419438000

This PR implements the mapping of user groups provided by OIDC providers
to orgs teams in Gitea. The main part is a refactoring of the existing
LDAP code to make it usable from different providers.

Refactorings:
- Moved the router auth code from module to service because of import
cycles
- Changed some model methods to take a `Context` parameter
- Moved the mapping code from LDAP to a common location

I've tested it with Keycloak but other providers should work too. The
JSON mapping format is the same as for LDAP.


![grafik](https://user-images.githubusercontent.com/1666336/195634392-3fc540fc-b229-4649-99ac-91ae8e19df2d.png)

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/auth/common.go

@@ -0,0 +1,22 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package auth
+
+import (
+	"code.gitea.io/gitea/modules/json"
+	"code.gitea.io/gitea/modules/log"
+)
+
+func UnmarshalGroupTeamMapping(raw string) (map[string]map[string][]string, error) {
+	groupTeamMapping := make(map[string]map[string][]string)
+	if raw == "" {
+		return groupTeamMapping, nil
+	}
+	err := json.Unmarshal([]byte(raw), &groupTeamMapping)
+	if err != nil {
+		log.Error("Failed to unmarshal group team mapping: %v", err)
+		return nil, err
+	}
+	return groupTeamMapping, nil
+}