Disable `ensure_ascii` on `json.dumps`

Current output is unreadable in console if any language other than English is used in the messages.

Reproduction code:
```python
import logging
import ecs_logging

logger = logging.getLogger()
logger.setLevel(logging.INFO)
handler = logging.StreamHandler()
handler.setFormatter(ecs_logging.StdlibFormatter())
logger.addHandler(handler)

logger.info("안녕하세요")
```

Expected output:
```json
{
"@timestamp": "2025-06-20T06:39:24.589Z",
"log.level": "info",
"message": "안녕하세요",
"ecs.version": "1.6.0",
"log": {
"logger": "root",
"origin": {
"file": { "line": 10, "name": "" },
"function": ""
},
"original": "안녕하세요"
},
"process": {
"name": "MainProcess",
"pid": 302584,
"thread": { "id": 140083028806592, "name": "MainThread" }
}
}
```

Received output:
```json
{
"@timestamp": "2025-06-20T06:39:24.589Z",
"log.level": "info",
"message": "\uc548\ub155\ud558\uc138\uc694",
"ecs.version": "1.6.0",
"log": {
"logger": "root",
"origin": {
"file": { "line": 10, "name": "" },
"function": ""
},
"original": "\uc548\ub155\ud558\uc138\uc694"
},
"process": {
"name": "MainProcess",
"pid": 302584,
"thread": { "id": 140083028806592, "name": "MainThread" }
}
}
```

Empieza buscando en el proyecto json.dumps y el formateador utilizado por ecs_logging.StdlibFormatter. Ejecuta la reproducción de Python proporcionada para confirmar la salida actual con caracteres escapados y, después, verifica que los caracteres no ingleses aparezcan directamente en ambos campos de mensaje, mientras el resultado siga siendo JSON válido.

Escrito por el modelo de indexación a partir del texto del issue.