feat/fedex customer references full support

[ChrisNolan]

This change expands FedEx shipment customerReferences support from a single hardcoded invoice reference to full multi-type handling.

What changed

• Added FedEx customer reference builders in provider utils and wired them into shipment request creation.
• Implemented reference mapping for:
  • CUSTOMER_REFERENCE (from shipment reference)
  • INVOICE_NUMBER (from customs invoice, with options fallback)
  • DEPARTMENT_NUMBER (from options)
  • P_O_NUMBER (from options)
  • RMA_ASSOCIATION (from options)
• Applied FedEx length constraints per reference type (including conservative CUSTOMER_REFERENCE limit for Ground compatibility).
• Removed incorrect PO mapping from package reference values.

What is now available (was not before)

• FedEx requests now send multiple valid customer reference types in the correct shipment sections (commercial invoice and package line items), instead of only a hardcoded invoice reference.
• API callers can now populate FedEx-specific references via shipment options (for example fedex_po_number, fedex_department_number, fedex_rma_association) and have them forwarded to FedEx correctly.
• Added focused unit coverage for reference construction/collection plus regression validation in FedEx shipment tests.

[vercel]

@ChrisNolan is attempting to deploy a commit to the karrio Team on Vercel.

A member of the Team first needs to authorize it.

[andersonvom]

This is great! I was about to put together a PR to address this very issue, but you beat me to it 🥳