posixGroup membership queries from Domain

`GitHub::Ldap::Domain#is_member?` doesn't correctly validate group membership for posixGroups due to some assumptions by the `Filter#member_filter` about the DN.

This changes the `Filter#member_filter` method to accept a `Net::LDAP::Entry` object and maps the correct group type with the correlated member attr (which, for posixGroup, is the member entry's CN).

Actually meant to highlight this part of the chunk. Here we're explicitly adding the `memberUid` conditions on top of the existing membership names.

Also updated to accept the `uid_attr` as a parameter (in case `@ldap` isn't available, for instance).