Boards API: retrieve project and milestone via API

Description

Projects can have multiple issues boards, and each issue board can be associated with a milestone. At the moment it is not possible to get this information through the Boards API.

Proposal

Boards API should be extended by the possibility to retrieve this information:

{
  "id": 1
  "project": {
    "id": 45,
    ...
  },
  "milestone": {
    "id": 20,
    "label": "v9.0"
    ...
  }
  "lists": [
    {
      "id": 1,
      "label": {
        "id": 3, 
        "name": "Bug", 
        "color": "#0ff00f", 
        "description": "Bugs"
      }
      "position": 2
    }
  ]
}

Links / references

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1150

cc @smcgivern