MM-16861: Support Guest Authentication via AD/LDAP (#12690)

* Add config settings for LDAPSettings GuestFilter

* make error unique

* Update model/config.go

Co-Authored-By: Martin Kraft <martin@upspin.org>

* add LdapSetting isempty_guest_attribute to diagnostics.go

model/config.go

@@ -1687,6 +1687,7 @@ type LdapSettings struct {
 	// Filtering
 	UserFilter  *string
 	GroupFilter *string
+	GuestFilter *string
 
 	// Group Mapping
 	GroupDisplayNameAttribute *string
@@ -1758,6 +1759,10 @@ func (s *LdapSettings) SetDefaults() {
 		s.UserFilter = NewString("")
 	}
 
+	if s.GuestFilter == nil {
+		s.GuestFilter = NewString("")
+	}
+
 	if s.GroupFilter == nil {
 		s.GroupFilter = NewString("")
 	}
@@ -2784,6 +2789,12 @@ func (ls *LdapSettings) isValid() *AppError {
 				return NewAppError("ValidateFilter", "ent.ldap.validate_filter.app_error", nil, err.Error(), http.StatusBadRequest)
 			}
 		}
+
+		if *ls.GuestFilter != "" {
+			if _, err := ldap.CompileFilter(*ls.GuestFilter); err != nil {
+				return NewAppError("LdapSettings.isValid", "ent.ldap.validate_guest_filter.app_error", nil, err.Error(), http.StatusBadRequest)
+			}
+		}
 	}
 
 	return nil