[ES|QL][Kibana request] Return unique id by default

### Description

We would like to return by default the _id of the document or to describe it with a more generic way a unique identificator. It doesnt need to be the _id as this is something tied closer to indices and documents but something that can describe uniquely one row.

Atm we are using the`metadata _id` field but this is not very user friendly as the users need to type it on their own and is not always available. This is important for Discover as we can use this id to retrieve more information. For example:

- The user clicks a row, a flyout with the row details appears and at this point I want to run an ES|QL query in the background to retrieve more metadata (or run statistics etc) only for this row. I dont want to do this by default as it is not performant. I want to run these extra queries on user's demand.
- Some features such row selection, comparison, and expanded docs all silently lose state on refresh
- Deep linking to a document or specific documents is not possible, we are enabling this feature when the _id is given by the users at the query

This should not change the current behavior of ES|QL. I am expecting to be a Discover mode where we can get a little bit different defaults. I don't expect that this meta information will be important everywhere in Kibana (for example we don't need the id in a visualization).