XD-2121 Add LDAP authentication support

* Establish an xd.security.authentication.ldap configuration root
* Support for LDAP bind authentication with user direct bind and user search
* The intent is for various authentication managers to live in the `org.springframework.xd.dirt.server.security` package, be scanned and activated depending on the configuration properties
* Group search support
* Tests for bind and search-and-bind configuration scenarios

NOTES:

1. Group management, while supported, will have no effect on security, as roles re anot applicable ATM.
2. Password comparison support should make the subject of another JIRA/PR as it is a common feature across various configuration mechanisms.
The intent of using the scan is to allow for other classes similar to LdapAuthenticationConfiguration to live in that package, and to activate just one of them according to a configuration-based conditional.