Upstreaming LDAP plugin

[nebulon]

To support Radicale v3 we had to develop the LDAP plugin, since the plugin structure changed. We are now trying to upstream this to the well known radicale ldap plugin at https://github.com/marcoh00/radicale-auth-ldap/pull/13

If you are a python developer, you are welcome to join us to fixup the things left to make it whole