kafka streams headers in the produce step

I think it is not possible to define headers in the producer json. Something like this:
```json
"steps": [
{
"name": "produce_step",
"url": "kafka-topic:xxxx",
"operation": "produce",
"request": {
"records": [
{
**"headers": [
{"type": "aaa"}
],**
"key": "577152c4-26d2-4479-ae05-a7272fa81eea",
"value": "{\"eventId\":\"577152c4-26d2-4479-ae05-a7272fa81eea\"}"
}
]
},
"assertions": {
"status": "Ok"
}
```

Start by locating the Kafka produce-step handling and the JSON request model used for records. Check how producer record fields are mapped, then determine the expected header representation and add coverage showing that headers are accepted and sent with produced records.

Written by the indexing model from the issue text.