Add Support For Additional Referenced Documents

[mnin]

Introducing new element ram:AdditionalReferencedDocument under //rsm:CrossIndustryInvoice/rsm:SupplyChainTradeTransaction/ram:ApplicableHeaderTradeAgreement like:

<ram:AdditionalReferencedDocument>
  <ram:IssuerAssignedID>example.pdf</ram:IssuerAssignedID>
  <ram:TypeCode>916</ram:TypeCode>
  <ram:Name>example.pdf</ram:Name>
  <ram:AttachmentBinaryObject mimeCode="application/pdf" filename="example.pdf">
    JVBERi0xLjMKJcTl8uXrp/Og0MTGCjMgMCBvYmoKPDwgL0ZpbHRlciAvRmxh
    dGVEZWNvZGUgL0xlbmd0aCAxOTkgPj4Kc3RyZWFtCngBXY/NigJBDITv8xR1
    ...
    ZDAzOGI3PiBdID4+CnN0YXJ0eHJlZgo2MjYxCiUlRU9GCg==
  </ram:AttachmentBinaryObject>

to embed attachments like PDF documents into the XML.