Add a hasSubcontracting boolean and a subcontracting object in Award objects

[ColinMaudry]

TED section V.2.5) and eForms BG-180 use the same structure to describe the details of the subcontracted portions of an awarded contract.

TED V.2.5) :

Examples:

{
"awards": [
          {
            "id": "1",
            "hasSubcontracting": true,
            "subcontracting": {
              "value": {
                "amount": 28000,
                "currency": "EUR"
              },
              "details": "The painting and electricity tasks are subcontracted."
            }
          }
        ]
}

{
"awards": [
          {
            "id": "1",
            "hasSubcontracting": true,
            "subcontracting": {
              "percentage": 0.2,
              "details": "The painting and electricity tasks are subcontracted."
            }
          }
        ]
}

[jpmckinney]

When creating the extension, be sure to add minimum/maximum JSON Schema validations for percentage.

[jpmckinney]

Changing to a pair of minPercentage and maxPercentage fields to work for F15.

[jpmckinney]

Closed by https://github.com/open-contracting-extensions/ocds_subcontracting_extension