Support static nested groups in LDAP

This refers to groups that have member: <another group DN> as an attribute
- Add in a utility method in the SpringSecurityLdapTemplate to retrieve multiple attributes and their values from an LDAP record
- Make the DefaultLdapAuthoritiesPopulator more extensible
- Add an LdapAuthority object that holds the DN in addition to other group attributes
- Add a NestedLdapAuthoritiesPopulator to search statically nested groups
As discussed offline, I think it may be best to break this up into its own ticket since we will likely want to back port it. I created https://jira.spring.io/browse/SEC-2691 for this purpose.