Define default response fields

[jordanpadams]

Acceptance criteria:
Check that the default fields are described in the specification:

1. https://github.com/NASA-PDS/pds-api/blob/master/docs/spec/pds-api-specification.md
2. and online https://nasa-pds.github.io/pds-api/

[tloubrieu-jpl]

Default fields are named universal or base attributes.

A first list agreed on during API workng group meeting on 11/04/2020 is:
{
"id": "string",
"type": "string",
"title": "string",
"description": "string",
"start_date_time": "string",
"stop_date_time": "string",
"observing_system_components": [
{
"title": "string",
"ref": "string",
"type": "string",
"description": "string"
}
],
"targets": [
{
"title": "string",
"ref": "string",
"type": "string",
"description": "string"
}
],
"metadata": {
"creation_date_time": "string",
"update_date_time": "string",
"version": "string",
"label_url": "string"
},

• a reference to mission context product (investigation area in the pds4 information model)