Add _id to the result of helpers.search

[ramikg]

Overview

This PR adds the _id field to the object returned by helpers.search.

Motivation

A vast majority of my use cases require knowing the IDs of the documents being fetched, which is why I end up using instead the regular search function instead of the helpers.search handy helper.

Additional considerations

At first I thought about adding an extra parameter to the function (extraOptions?), but after some consideration, I'm not sure that changing the existing behavior would be disruptive.
Still, I don't mind re-implementing this feature in any other manner.

Some notes

1. _id is an illegal field name, meaning _source will never include it.
2. Here is the @es_quirk I'm referring to in the comments (exists since version 8.14.0):

   /**
     * @es_quirk '_id' is not available when using 'stored_fields: _none_'
     * on a search request. Otherwise the field is always present on hits.
   */
   

   (In the code I rather not quote it.)

[cla-checker-service]

💚 CLA has been signed

[JoshMock]

This makes complete sense. Thanks for the contribution!

[ramikg]

Thank you!