[mongo] support keys with periods in them in JSON mode

Right now a document like 

```
{
  "_id": {
    "$oid": "689b9b3411b7facf94763866"
  },
  "field.with.dots": "value",
  "normal_field": "value2"
}
``` 

gets inserted as 

```
 {"_id":"689b9b3411b7facf94763866","field":{"with":{"dots":"value"}},"normal_field":"value2"}
```

because of how ClickHouse JSON works