Enforce two-factor auth (2FA: TOTP or WebAuthn) (#34187)

Fix #880

Design:

1. A global setting `security.TWO_FACTOR_AUTH`.
* To support org-level config, we need to introduce a better "owner
setting" system first (in the future)
2. A user without 2FA can login and may explore, but can NOT read or
write to any repositories via API/web.
3. Keep things as simple as possible.
* This option only aggressively suggest users to enable their 2FA at the
moment, it does NOT guarantee that users must have 2FA before all other
operations, it should be good enough for real world use cases.
* Some details and tests could be improved in the future since this
change only adds a check and seems won't affect too much.

---------

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

services/auth/source/pam/source.go

@@ -17,12 +17,10 @@ import (
 
 // Source holds configuration for the PAM login source.
 type Source struct {
-	ServiceName    string // pam service (e.g. system-auth)
-	EmailDomain    string
-	SkipLocalTwoFA bool `json:",omitempty"` // Skip Local 2fa for users authenticated with this source
+	auth.ConfigBase `json:"-"`
 
-	// reference to the authSource
-	authSource *auth.Source
+	ServiceName string // pam service (e.g. system-auth)
+	EmailDomain string
 }
 
 // FromDB fills up a PAMConfig from serialized format.
@@ -35,11 +33,6 @@ func (source *Source) ToDB() ([]byte, error) {
 	return json.Marshal(source)
 }
 
-// SetAuthSource sets the related AuthSource
-func (source *Source) SetAuthSource(authSource *auth.Source) {
-	source.authSource = authSource
-}
-
 func init() {
 	auth.RegisterTypeConfig(auth.PAM, &Source{})
 }