Read Nested Property Mappings

[JoonLeeNIH]

The collectPage() method renames root-level properties in a document to match the field names used by the backend's GraphQL schema. This PR enables it to rename properties nested one level deep as well.
Supports this Jira ticket: https://tracker.nci.nih.gov/browse/C3DC-1263

[JoonLeeNIH]

Pushed new commits for restructuring the property mappings between GraphQL and Opensearch. I propose this new structure for property mapping:

[
  {
    'gqlName': 'age_at_diagnosis',
    'osName': 'age_at_diagnosis',
  },
  {
    'gqlName': 'participant',
    'osName': 'participant',
    'nested': [
      {
        'gqlName': 'id',
        'osName': 'id',
      },
      {
        'gqlName': 'race',
        'osName': 'race_str',
      },
    ],
  },
  {
    'gqlName': 'treatments',
    'osName': 'treatments',
    'nested': [
      {
        'gqlName': 'id',
        'osName': 'id',
      },
      {
        'gqlName': 'treatment_agent',
        'osName': 'treatment_agent',
      },
    ],
  },
]

[huuaho]

LGTM