Support binding to LDAP server without requiring credentials

See https://forum.xwiki.org/t/ldap-authenticator-use-current-credentials/3636

Scenario: XWiki application server is using SSO and running in user context that already has permission to bind to LDAP server.

In this scenario, there is no need to authenticate to the LDAP server. You can just run a query directly.

This is a request to add a configuration option to bind without using credentials.

Thank you for your consideration.