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

[pchila]

I think this is probably a case of #2522

Yes it is very similar to what described in a comment of the same issue #2522 (comment)

My idea here is to maintain the flexibility in input by adding support for the float64 type (I am not using validation from ucfg or anything of the sort so I can switch on the type of the value)