SQL: consider _source instead of docvalue_fields for most of the fields' retrieval

[astefan]

At the moment, ES SQL uses docvalue_fields for retrieval of all the requested fields in a query. This, though, has the downside mentioned in elastic/kibana#35489.

The alternative is to load the _source and extract the needed fields from there.

On one hand, the difficulty here is especially relevant for date and numeric fields where ES SQL needs to do the same parsing ES is doing when indexing a document so that the user is presented with the same value it would get back from Elasticsearch itself.

On the other hand, if one field only is already taken from _source, we should take all of them from _source since the _source is already loaded and built as JSON and the computational effort to re-parse the numeric and date fields is worth it.

Or, as a second alternative, maybe get fields other than dates from _source and the dates from docvalues_fields and skip re-parsing the dates.

[elasticmachine]

Pinging @elastic/es-search

[astefan]

7.x (7.4.0): 2633d11
master (8.0.0): 8bf8a05