Sending request headers

We have a use case where it is required to set certain request headers, based on a key or value from Kafka.

Would it be possible to add this feature? I can see several ways to do it.

1. Automatically use fields in the record key as headers. This may make sense but it is prone to sending unnecessary headers.
2. Specify a certain prefix for headers in the record key, e.g. `http_header_*`. When a field matches this pattern, send it as a request header, removing the `http_header_` prefix.
3. Instead of sending the value as a request body, use nested fields `"headers"` and `"body"` to describe the payload.