ECS schema validation in logger

Hi,

This is more of a question than an issue. Is there a validation feature in `ecs-logging-python` library? I wish to avoid mapping conflict later on during the application on-boarding process, and I wish to make the app ECS compatible from it's very inception.

Currently adding extra fields is simple:
```python
logger.debug(f"Creating directory. {x}", extra={"not_an_ecs_field.method": "get", "source.ip": 22})
```

I have added two fields, one of which is not really an ECS schema field while the other has an invalid data type. I was wondering if I could pass in the ECS schema as a JSON and have the logger do the validation. I read an option to pass in a validator [here](https://github.com/elastic/ecs-logging-python/blob/master/ecs_logging/_stdlib.py#L112), but not sure what it expects.

检查 ecs_logging/_stdlib.py 第 112 行附近的内容以及 issue 中提到的 validator 选项，然后将其与示例中的 extra fields 进行比较。完成这项工作需要确定是否支持 ECS schema 验证，并定义预期的 validator 输入或验证功能的范围。

由索引模型根据 Issue 内容生成。