Capture cohort information in study index

[DSuveges]

As the GWAS Catalog team curates cohort information, which is also available in the downloadable files, the schema and the ingesting logic is extended with this new field.

    {
      "name": "cohorts",
      "type": {
        "type": "array",
        "elementType": "string",
        "containsNull": true
      },
      "nullable": true,
      "metadata": {}
    }

This filed needs to be populated by the datasource specific parsers:

• For Finngen studies this field can be array(lit('FINNGEN')).alias('cohorts)
• For GWAS Catalgog studies this field needs to be split by ' - '