how to prefix structlog log value with "custom." or route them to a certain ecs field

Hi,
according to the ecs documentation non ecs standard fields should go under "custom."
Also I might log data that should go to a certain ecs field

but i'm not sure how to configure something like this in structlog/ecs-logging.

for example this:
```python
req_id = uuid()
...
logger.info("something happend", foo="123", request_id=req_id)

```
should end up being something like this:
```json
{
"http.request.id" : "",
"message": "something happend",
"custom.foo" : 123
...
}
```