Update message shim to handle inserting headers on n messages in a producer

[bizob2828]

Description

Kafkajs allows you to send multiple messages in 1 send call. Our message-shim only adds headers to one message. Our current approach is to just DT headers to the first message.

Acceptance Criteria

As a kafkajs user I want the ability to include DT headers when sending multiple messages in a producer.send or producer.sendBatch call.

Additional context

Here's a poc branch that adds a new function to handle returning all the headers.

[workato-integration]

https://new-relic.atlassian.net/browse/NR-277167