Is your feature request related to a problem? Please describe.
Currently headers field is for static values.
We need possibility to put value from event data in header when using HTTP trigger.

Describe the solution you'd like
Given we have event like this:

    {
        "context": {
          "type": "type_of_event_source",
          "specversion": "cloud_events_version",
          "source": "name_of_the_event_source",
          "id": "unique_event_id",
          "time": "event_time",
          "datacontenttype": "type_of_data",
          "subject": "name_of_the_configuration_within_event_source"
        },
        "data": {
          "header": {/* the headers from the request received by the event-source from the external entity */},
          "body": { "userId":"2" },
        }
    }

I wanna have something like this:

          headers:
            MY_HEADER_1: {{ contextKey: id }}
            USER_ID: {{ dataKey: body.userId }}

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
This will be useful when you need to have guarantees that events generated for user with id number 1 will be received only by one instance of other service. We can calculate hash based on USER_ID and route requests to specific instance.

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