Allow setting returned fields in search_request.xml

### Summary (*)

It's not possible to customize fields returned by search engines (for example Elasticsearch) in `search_request.xml`.

### Examples (*)

In the case of Elasticsearch, `Magento\Elasticsearch\SearchAdapter\Query\Builder` forces the returned fields to be `['_id', '_score']`. This is OK in the case of product search, because you can fetch the remaining data through the product repository using the returned product IDs. But in simpler search cases, it would be nice to return the fields directly from the search index.

### Proposed solution

I could provide a PR for this, but is there an explicit reason this is not possible at the moment? Maybe some incompatibility issue between search engines, which prevents this being part of the `search_request.xml` abstraction?

Start with search_request.xml and Magento\Elasticsearch\SearchAdapter\Query\Builder, where the issue says returned fields are forced to _id and _score. Compare how other search engines handle requested fields and determine what configuration is compatible across the search abstraction; done means supported fields can be configured without breaking product search.

Written by the indexing model from the issue text.