Kafka Connector - Add ability to read and write headers

[rd-andreas-beckmann]

Is your feature request related to a problem? Please describe.

Headers are used to communicate meta information (e.g. correlation ids) between services. This requires the ability to read and write header values, which is currently not supported within the out-of-the-box kafka connector.

Describe the solution you'd like

Depending on the inbound/outbound-connector, following solution would be desirable:

Inbound
Similarly to the key and value properties of the received object, an additional header attribute containing the header information as json object would resolve the need. This solution should be backwards compatible and not require any changes to preexisting integrations.

{
    "key": "...",
    "rawValue": {[...]},
    "value": {[...]},
    "headers":{
        "headerKey1":"headerValue"
        "headerKey2":"headerValue2"
    }
}

Outbound

Adding a headers property within the message group (similarly to the key and value properties), would resolve the need. Making the property optional, would ensure backwards compatibility. Setting the headers as json Object Strings

{
     "headerKey1":"headerValue"
     "headerKey2":"headerValue2"
}

would enable setting the values dynamically as part of a workflow.

Describe alternatives you've considered
Not applicable

Additional context
If needed, a mock of the outbound template can be created.

Additional ticket: https://jira.camunda.com/browse/SUPPORT-18647