Add a project-level milestones array

[duncandewhurst]

To allow publishers to disclose key project dates for which there isn't a specific field in OC4IDS, e.g. the operation and maintenance end date discussed in #252

[duncandewhurst]

SIF's SOURCE platform also includes the following project-level milestones which are not covered by explicit fields in OC4IDS:

• Planning start (target and actual)
• Planning end (target and actual)

ANZIP includes the following project-level milestones which are not covered by explicit fields in OC4IDS

• Construction commencement (expected and actual)

[duncandewhurst]

Example (shared in CRM-6710):

{
  "projects": [
    {
      "milestones": [
        {
          "id": "1",
          "title": "Operation and Maintenance Period End Date",
          "description": "The end of the technical operation, maintenance and/or provision of services relating to the infrastructure asset",
          "dueDate": "2030-03-14T17:00:00Z",
        },
        {
        }
      ]
    }
  ]
}

[jpmckinney]

Closed by #426 (and again in #456)