QL: integrate high-level field retrieval support in SQL and EQL

Following the merge of [high-level field retrieval in _search](https://github.com/elastic/elasticsearch/pull/60100) (docs [here](https://www.elastic.co/guide/en/elasticsearch/reference/7.11/search-fields.html#search-fields-param)), we should switch from extraction from `_source` and `docvalue_fields` to using `fields` option.

 