Additional query attributes option for Member search strategy

Adds an option for the Member search strategy to include attributes in member queries. This allows us to specify attributes like `objectClass` in addition to the required *member* attributes for other purposes (like filtering out groups by `objectClass`).

cc @jch 

Both, really. `member` and `uniqueMember` are attributes of the `groupOfNames` and `groupOfUniqueNames` object classes (and Active Directory's `group` category), and `memberUid` is for the `posixGroup` object class.

Hardcoding these here (and elsewhere) is a smell. Addressing that smell is a fairly large engineering undertaking that isn't possible right now. I have some ideas I'd like to run by you, though, for the future.