Update issueDate and all credentialSubject date fields to date-time
fix: update issueDate and all credentialSubject date fields to date-time across all schemas (closes #37)
All date fields in credentialSubject that map to UNCEFACT DateTime vocabulary terms were typed as format:date. Updated to format:date-time consistently across all 10 document schemas and their example files.
Affected schemas: commercial-invoice, bill-of-lading, certificate-of-origin, preferential-certificate-of-origin, customs-declaration, insurance-certificate, packing-list, purchase-order, shippers-letter-of-instructions, warehouse-receipt.