Fix failure_threshold parsing when value is coming from fleet

[pchila]

While investigating #6049 I tried to set the failure_threshold using fleet override API, however this is parsed as a float64 by agent which is not supported at the moment, for example:

PUT kbn:/api/fleet/agent_policies/<policy_id>
{
   "name": <policy_name>,
   "namespace": "default",
   "overrides": {
       "agent": {
            "monitoring": {
                "failure_threshold": 5
            }
       }
   }
}

Optionally figure out how to reset the failure_threshold to default from fleet, agent should probably check if there's a monitoring section coming from fleet but no failure_threshold key and reset its monitoring failure_threshold

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[cmacknz]

I think this is probably a case of #2522