[Search] Implement a background search service

[lizozom]

Part of #61738

The background search service is responsible for

• Tracking in memory mappings of sessionId to multiple searchIds (also tracking the user and request that generated that search.
• Clearing out those mappings if they are stale.
• Monitoring the existence of a saved object for that sessionId (indicating a background search was requested) and storing the search IDs into it.
• Restore an searchId for a given sessionId from a saved object.

Tasks:

• Define a saved object to represent a background search
• Define a BackgroundSearch service

        interface BackgroundSearchService {
            /* Track a `searchId` in memory or on a background search saved object (if exists). */
            async trackId: (userId: string, sessionId: string, requestParams: any, searchId: string) => void;
            /* Get a `searchId` from a background search saved object */
            async getId(userId: string, sessionId: string, requestParams: any): string | undefined;
            /* Create a background search saved object for a given session. */
            async store(sessionId: string) => void;
        }

• Define a route internal\search\bg (??) where client can send a sessionId and optionally a list of searchIds. Invoke backgroundSearch.store(sessionId) and trackId for each searchId.

[elasticmachine]

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

[lizozom]

#84225
#84837
#84975
#85350