Expose Group.group? for testing if entry is a group

[mtodd]

Minimal change to expose GitHub::Ldap::Group.group?(entry) to check if the objectClass matches any known group types (group, groupOfNames, groupOfUniqueNames).

cc @jch

[jch]

@mtodd The class method makes sense to me since the method doesn't depend on any instance state. The group? instance method was previously documented as internal, how do you feel about removing the instance method and just calling the class method when we need it? I only see one caller in this file.

[mtodd]

@jch was tempted to make that change but decided to address that separately. I think you're right, though, it's way more useful like this.