Add optional identifier to data center level

[chad-earthscope]

We should consider adding an optional identifier to the datacenter level to allow data centers to indicate foreign identifiers in other systems, e.g. re3data/datacite, DOI, whatever.

For example:

{
  "datacenter": {
    "IRISDMC": {
      "identifier": [
        {
          "identifier": "r3d100010268",
          "identifierType": "re3data"
        }
      ]
    }
  }
}