Performance with Accessions /list as CSV

Using the CSV format to retrieve large selections (pages) using the /list endpoint appear to be very slow (see genesysr#2 (comment 53485)).

We should explore two things:

1. Adding support to exclude fields (we now have select to include fields)
2. Not serializing things with flatMap that are not selected.

The response is now very broad and includes very deeply nested entities. We should explore what can be excluded.