feat(apisix-standalone): add server field to sync result

[bzp2010]

Description

We have implemented single-batch synchronization for multiple servers on the apisix-standalone backend. Consequently, the final synchronization result will pertain to all servers collectively, and currently, no field indicates which server succeeded or failed.

This PR adds an optional server field to the synchronization result and populates it in the apisix-standalone backend.

{
  "status": "partial_failure",
  "total_resources": 4,
  "success_count": 2,
  "failed_count": 2,
  "success": [
    {
      "server": "http://127.0.0.1:29180",
      "event": {},
      "synced_at": "2025-08-26T10:45:20.000Z"
    },
    {
      "server": "http://127.0.0.1:19180",
      "event": {},
      "synced_at": "2025-08-26T10:45:20.000Z"
    }
  ],
  "failed": [
    {
      "server": "http://invalid.host:19180",
      "event": {},
      "failed_at": "2025-08-26T10:45:20.623Z",
      "reason": "getaddrinfo ENOTFOUND invalid.host"
    },
    {
      "server": "http://invalid.host:29180",
      "event": {},
      "failed_at": "2025-08-26T10:45:20.623Z",
      "reason": "getaddrinfo ENOTFOUND invalid.host"
    }
  ]
}

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible