Provide a way to only extend async_search keep_alive

[Dosant]

Description

Currently, in Kibana we extend async searches using the keep_alive parameter of GET async_search request.

Kibana uses search polling to support long-running searches. It polls with keep_alive:1m until the search completes or the user leaves.

However, as part of the search-session feature, we extend searches that belong to a saved search session to a 1w. Since in parallel there might be a search polling process that is using keep_alive:1m we might hit a race condition where some searches are overwritten with 1m instead of previously set 1w.

We have and can work around this race condition by some client-side management overhead, but in general, it would be useful to us to have an API that allows us to only extend the keep_alive.

Please evaluate if this is a worthy addition to the API or feel free to close otherwise.

Some previous related work on this:

• Older issue in Kibana where this came up during initial implementation [Discuss][Search Sessions] Synchronizing async_search keepslive updates kibana#88599 but then was resolved by Architectural changes in Kibana that we want to reconsider now
• Related ES pull request that was reverted Extend async search keep alive #67877

[elasticmachine]

Pinging @elastic/es-search (Team:Search)

[javanna]

We have no plans to work on this, please reach out if this is a blocker on your end.