Adds a highly configurable LDAP-Adapter

This PR replaces the currently available LDAP-Adapter with a highly configurable one. 

The new LDAP-Adapter can be used as authentication adapter for any LDAP-Server. Usage and Configuration are described in the acompanying LDAP_README.md file

Applying this LDAP-Adapter is a clear BC-Break as current configurations will not work any more. On the other hand, after an update the administrator has to reconfigure the LDAP-Adapter in any case as it has been replaced during the upgrade. Therefore that should not be an issue!