Implement LDAP ppolicy login messages

Situation
LDAP as authentication source in SimpleSAMLphp
LDAP password policy overlay
When using ppolicy password aging, no messages are shown when logging in with SimpleSAMLphp.
If the password is almost expired (pwdExpireWarning) a warning is sent by LDAP.

New developments
That warning can be caught as of php 7.3 via the LDAP control LDAP_CONTROL_PASSWORDPOLICYREQUEST.
See php manual pages LDAP controls and Example #1 Bind with ppolicy information

Solution
If ppolicy handling is enabled via new ldap module config, show the message to the user before continuing to the SP. The message shown should be configurable, for example to set a link where users will be able to change their password.

首先跟踪 LDAP 身份验证流程，并找到可能启用 ppolicy 处理的模块配置。阅读 PHP 的 LDAP controls 文档和 bind 示例，然后在继续处理 service provider 之前，确定如何显示可配置的警告消息；当启用该选项时，关于密码即将过期的警告能够传达给用户，即表示完成。