Onboard Error Count Threshold rule type with FAAD (#179275)

Towards: #169867

This PR onboards the Error Count Threshold rule type with FAAD.

### To verify

1. Run the following script to generate APM data:
```
node scripts/synthtrace many_errors.ts --local --live
```

2. Create an error count threshold rule.
Example:
```
POST kbn:/api/alerting/rule
{
  "params": {
    "threshold": 25,
    "windowSize": 5,
    "windowUnit": "m",
    "environment": "ENVIRONMENT_ALL"
  },
  "consumer": "alerts",
  "schedule": {
    "interval": "1m"
  },
  "tags": [],
  "name": "testinggg",
  "rule_type_id": "apm.error_rate",
  "notify_when": "onActionGroupChange",
  "actions": []
}
```
3. Your rule should create an alert and should saved it in
`.internal.alerts-observability.apm.alerts-default-000001`
Example:
```
GET .internal.alerts-*/_search
```
4. Recover the alert by setting `threshold: 10000`

5. The alert should be recovered and the AAD in the above index should
be updated `kibana.alert.status: recovered`.