GET /v2/custodians

List all available custodians.

Note: marked "not yet available" in the public docs. May 404 in
production.

Path params: none.
Query params: none.

Response (200 OK):
{
  "data": [
    {
      "id": "string",
      "type": "custodian",
      "attributes": {
        "identifier": "string — e.g. nordea-dk",
        "name":       "string — display name",
        "country":    "string — ISO 3166-1 alpha-2"
      }
    }
  ]
}