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).

Might need to make this pull from configuration, since `memberUid` might not only be `cn`.

For example, look at [`PosixGrou#search_members_by_uids`](https://github.com/github/github-ldap/blob/9040776fef2b2dc193dbffe8cc54ae8cd3a28451/lib/github/ldap/posix_group.rb#L76).