Usage of x-opaque-id for multiple values

[lizozom]

Elasticsearch supports passing in the x-opaque-id header to the tasks executing that search, for identification.

However, at the moment, we don't have a proper way of using it for any purpose (for example to associate search tasks with a Search Session), as the header is being over-written by core, for audit logging pirposes.

Ideally, we should be able to pass in and store multiple values using the x-opaque-id, so I believe we should discuss the details and implications of this approach.

[lizozom]

Other related issues:

#41530
#16493
#60122