Add ExternalService for unknown Service elements

[donaldgray]

Added V2 and V3 ExternalService classes. These are used as fallbacks if we need to deserialize an IService and don't have a type to deserialize too. These both inherit from ResourceBase.

The anticipation is that this will be used to map unknown services such as

"services": [
  {
    "id": "https://iiif.wellcomecollection.org/presentation/b18035723#tracking",
    "type": "Text",
    "profile": "http://universalviewer.io/tracking-extensions-profile",
    "label": {
      "en": [
        "Format: Monograph, Institution: n/a, Identifier: b18035723, Digicode: diggenetics, Collection code: n/a"
      ]
    }
  },
  {
    "id": "https://iiif.wellcomecollection.org/presentation/b18035723#timestamp",
    "type": "Text",
    "profile": "https://github.com/wellcomecollection/iiif-builder/build-timestamp",
    "label": {
      "none": [
        "2021-04-29T21:58:28.9247406Z"
      ]
    }
  },
  {
    "id": "https://iiif.wellcomecollection.org/presentation/b18035723#accesscontrolhints",
    "type": "Text",
    "profile": "http://wellcomelibrary.org/ld/iiif-ext/access-control-hints",
    "label": {
      "en": [
        "open"
      ]
    }
  }
]

from https://iiif.wellcomecollection.org/presentation/b18035723

[donaldgray]

Note - I'll add some additional tests around this.