Support for Message Subject in aws_sns

[obbteam]

Well currently Message Subject is not supported by the aws_sns output module. The only way is to send it via Message Attributes as metadata or the actual Message Payload but it looses the point.

Would be nice to have something like this:

        output:
          aws_sns:
            topic_arn: ${TOPIC_ARN}
            region: ${AWS_REGION}
            endpoint: ${SNS_ENDPOINT:}
            subject: SOME SUBJECT <------------------------
            credentials:
              id: ${AWS_ACCESS_KEY_ID:}
              secret: ${AWS_SECRET_ACCESS_KEY:}

[berkinakaydin]

We also need the same feature, would be great to have

[Thijmen]

Seems like #3960 supports this - feel free to add a thumbs up