Improve syntax for conformance overlay

[mitfik]

Currently conformance is defined as follow (see spec):

{
  "digest": "EIP9FA6J_fvw0wjnvjPVQqkoYol627D-NdvkhptxfyDM",
  "capture_base": "EVyoqPYxoPiZOneM84MN-7D0oOR03vCr5gg1hf3pxnis",
  "type": "spec/overlays/conformance/2.0.0",
  "attribute_conformance": {
    "dateOfBirth": "M",
    "documentNumber": "M",
    "documentType": "M",
    "fullName": "M",
    "height": "O",
    "issuingState": "M",
    "photoImage": "M",
    "sex": "M"
  }
}

This can be reduced to:

{
  "digest": "EIP9FA6J_fvw0wjnvjPVQqkoYol627D-NdvkhptxfyDM",
  "capture_base": "EVyoqPYxoPiZOneM84MN-7D0oOR03vCr5gg1hf3pxnis",
  "type": "spec/overlays/conformance/2.0.0",
  "attribute_conformance": [
    "dateOfBirth",
    "documentNumber",
    "documentType",
    "fullName",
    "issuingState",
    "photoImage",
    "sex"
  }
}

Means only mandatory attributes are listed in a form of array, similarly as defined in sensitive overlay.

Reason for it is that by default any attribute is optional if not specify otherwise (or at least this can be assumption as the state is not defined clearly). Means if you specify few attributes mandatory others automatically can be assumed optional without specifying it.

Interested of others points of view on that.