Fix background API calls that extend session idle timeout

[jportner]

Kibana version: Behavior is observed since session idle timeout was introduced, at least as early as 7.6

Describe the bug: Some Kibana pages periodically make background XHR calls that are detected as user activity, thus unintentionally extending the idle session timeout.

Steps to reproduce: See #79889.

Expected behavior: Sessions should time out as expected.

Any additional context: This issue is for a short-term fix, which is to find the offending XHR calls and add the kbn-system-request header to each of them. The long-term fix in the linked issue is to implement a less error-prone method of detecting user (in)activity.