Add Copy Number to Requests CSV

Purpose: To add missing column to the requests CSV. Spin off from UIREQ-114 so we are creating carry-over story.

Scenarios:

1. Scenario
   • Given the Requests screen (the list that results from a search within the Requests app)
   • When a user clicks export on the Requests (search results) screen
   • Then the following fields are exported as CSV for each request (new fields to be added as part of this story are indicated in blue with asterisk):
     • Request type
     • Request status
     • Request expiration date
     • Hold shelf expiration date
     • Position in queue
     • Item barcode
     • Title
     • Contributor (formerly author)
     • Shelving location (Effective Location)
     • Call number
     • Enumeration
     • Copy number (currently known as "Piece identifier" on the backend)*
     • Item status
     • Current due date
     • Requester Name
     • Requester Barcode
     • Requester Patron group
     • Request Fulfillment preference
     • Pickup service point
     • Delivery address
     • Requester's proxy name
     • Requester's proxy barcode