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 

This existed before, so I'm not asking for a change, but are these standard LDAP attributes? Or are they attributes we expect when using this strategy?