ES|QL: expose the query transport action to improve usage

[ChrisHegarty]

This commits exposes the query transport action to improve usage. While one can perform all operations prior to this change, it has been suggested that adding the action would improve the symmetry of the API by allowing e.g.

client().execute(builder.action(), builder.request()).actionGet(30, SECONDS);

[ChrisHegarty]

/cc @jan-elastic

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[costin]

LGTM