Kafka input should be able to add topic name to the messages

## Expected Behavior

When declaring a kafka input, the topic to read can be set via a regex. There should:
a) be an option to store the actual topic name where a message comes from into the message
b) specify the fieldname where that should be stored

## Current Behavior

No topic is stored with the message

## Context

Allows for additional filtering / putting it into different streams etc without having to create multiple kafka sources