[Search][Session] Make sure relative timeranges are handled correctly

[lizozom]

The Kibana server is going to use the body of the search request to generate the hash of a Background Session.
For this reason, it is critical that the body of the search request is identical when it's created and restored.

In order to do that we need to make sure that:

• An absolute time is always saved into the restoreOptions of the BackgroundSession
• We'll have a second, originalOptions that will contain the time as was configured in the dashboard (absolute of relative).
• All panels of a dashboard are sent with the same time (which is also the same time in the BackgroundSession).

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[lizozom]

Resolved by #84405