Add automatic telemetry reporting for endpoint availability messages

[kzu]

When using our endpoint for flow data, WhatsApp for Business will automatically send endpoint availability updates (from pings and actual usage) stats which we should track and report as our own telemetry for unified tracking.

Docs: https://developers.facebook.com/docs/whatsapp/flows/guides/healthmonitoring

Example of a message:

{
  "entry": [
    {
      "id": "539235785933710",
      "time": 1755544869,
      "changes": [
        {
          "value": {
            "event": "ENDPOINT_AVAILABILITY",
            "message": "Flow endpoint availability successfully recovered. Users will now be able to open and use the flow.",
            "flow_id": "1987937931743377",
            "availability": 100,
            "threshold": 90,
            "alert_state": "DEACTIVATED"
          },
          "field": "flows"
        }
      ]
    }
  ],
  "object": "whatsapp_business_account"
}