Support search fields API in SearchSource

[kertal]

To allow SearchSource consumers like Discover to support runtime fields (#77107), SearchSource needs to allow usage of the search-fields API of Elasticsearch

https://www.elastic.co/guide/en/elasticsearch/reference/master/search-fields.html

In the SearchSource source code fields is already taken:

kibana/src/plugins/data/public/search/search_source/types.ts

Line 64 in 61951a5

fields?: NameList;

And this fields target stored_fields, this can't just be switched to the ES search fields, since it's not compatible.

kibana/src/plugins/data/public/search/search_source/search_source.ts

Lines 494 to 505 in 61951a5

	if (fields) {
	// filter out the docvalue_fields, and script_fields to only include those that we are concerned with
	body.docvalue_fields = filterDocvalueFields(body.docvalue_fields, fields);
	body.script_fields = pick(body.script_fields, fields);
	// request the remaining fields from both stored_fields and _source
	const remainingFields = difference(fields, keys(body.script_fields));
	body.stored_fields = remainingFields;
	setWith(body, '_source.includes', remainingFields, (nsValue) =>
	isObject(nsValue) ? {} : nsValue
	);
	}

In the POC of #75407 this was solved by introducing a new SearchSource fields called fieldsApi, but this should be solved in a developer friendly, documented and tested way, not as a workaround.

[kertal]

Closing in favor of #77241