MgrStatMonitor: add config observer

MgrStatMonitor: add config observer

This commit adds a config observer to MgrStatMonitor so we
can track when a user enables/disables enable_availability_tracking
config option. The time difference between disabling and then
enabling the config option will be used to offset the uptime
and/or downtime from the availability score feature.

Fixes: https://tracker.ceph.com/issues/71494
Signed-off-by: Shraddha Agrawal <shraddhaag@ibm.com>
(cherry picked from commit c318f80ee2eeefbba9865f07026e18c313cba558)