Expose value-key on nuxeo-document-attachments

This is required to fix the upload to multivalued Blob properties whose XSD definition does not follow the one of "files:files"

Example of files:files

• the XSD definition is

    <xs:element name="files" type="nxs:files" />
  
    <xs:complexType name="files">
      <xs:sequence>
        <xs:element name="item" type="nxs:file" minOccurs="0"
          maxOccurs="unbounded" />
      </xs:sequence>
    </xs:complexType>
  
    <xs:complexType name="file">
      <xs:sequence>
        <xs:element name="file" type="nxs:content" />
      </xs:sequence>
    </xs:complexType>
  

• The JSON payload is

    "properties": {
      "files:files": [
        {
          "file": {
            "upload-batch": "batchId-1f58f604-f180-45ee-a8ba-1342e51c204d",
            "upload-fileId": "0"
          }
        }
      ]
    }
  

Example of a multi Blob property defined in Studio

• the XSD definition is

    <xs:complexType name="blobList">
      <xs:sequence> 
        <xs:element name="item" type="nxs:content" minOccurs="0" maxOccurs="unbounded"/>
      </xs:sequence> 
    </xs:complexType>  
    
    <xs:element name="attachments" type="nxs:blobList"/>
  

• The JSON payload should be

   "properties": {
      "file_schema:attachments": [
          {
            "upload-batch": "batchId-1f58f604-f180-45ee-a8ba-1342e51c204d",
            "upload-fileId": "0"
          }
      ]
    }
  

Because WebUI cannot inspect any property to know which type of definition is used, the easiest solution is to expose a new property for nuxeo-document-attachments to set "value-key" at https://github.com/nuxeo/nuxeo-web-ui/blob/maintenance-3.0.x/elements/nuxeo-document-attachments/nuxeo-document-attachments.js#L78