Search indexing only supports two levels of JSON nesting

[dafeder]

Current Behavior

If we have a dataset schema that contains more than two levels of nested objects, adding a field to the index from deeper in the tree will produce an error.

Expected Behavior

We should be able to add a field from anywhere in the dataset schema to the DKAN search index without issue.

Steps To Reproduce

1. Build a site and replace the dataset schema with this file
2. POST this dataset:

{
    "title": "Test dataset",
    "description": "This is a test dataset",
    "accessLevel": "public",
    "modified": "2019-01-01",
    "keyword": ["test", "dataset"],
    "distribution": [
        {
            "title": "Test distribution",
            "format": "text/csv",
            "downloadURL": "https://example.com/test.csv",
            "dctconformsto": [
                {
                    "dct_standard": "https://www.w3.org/TR/vocab-dcat-2/"
                }
            ]
        }
    ]
}

3. Go into the search API index configuration, and add the field "dctconformsto" to the index
4. Re-index

You will see an error that includes

Error: Object of class stdClass could not be converted to string in Drupal\search_api\Plugin\search_api\data_type\StringDataType->getValue() (line 23 of /var/www/html/docroot/modules/contrib/sea
rch_api/src/Plugin/search_api/data_type/StringDataType.php).

Environment

- OS:
- Node:
- npm:

Relevant log output

No response

Anything else?

See the discussion in #4180 where this was discovered. We need a more recursive kind of logic in Drupal\metastore_search\ComplexData\Dataset to allow adding fields that may be nested in objects nested in arrays nested in other objects, etc.