[Cases] Create API for retrieving all alerts for a case ID

[jonathan-buttner]

This issue is to track the work for creating an API within the cases plugin to retrieve all the alerts associated with a specific case ID.

The short term goal is to create an API that simply returns all of the alerts. In the future we can provide a more robust solution that allows doing search after or something similar.

For the implementation we can use what Larry suggested on the RBAC PR:

     const finder = soClient.createPointInTimeFinder<CaseUserActionAttributes>({
        type: CASE_USER_ACTION_SAVED_OBJECT,
        hasReference: { type, id },
        sortField: 'action_at',
        sortOrder: 'asc',
      });

      let result: Array<SavedObject<CaseUserActionAttributes>> = [];
      for await (const userActionSavedObject of finder.find()) {
        result = result.concat(userActionSavedObject.saved_objects);
      }
      return result;

Reference: #95058 (comment)

[elasticmachine]

Pinging @elastic/security-threat-hunting (Team:Threat Hunting)

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)