Investigate support for alternative serializers

[gkinsman]

Currently JustSaying is tightly coupled to the JustSaying wire format. This is problematic as the wider business begins to adopt alternative formats, such as cloud events. In addition, there is no native support for message attributes in the JustSaying format, which means that JS is also coupled to the SQS message format for these.

We need to do some exploration to discover how difficult it would be to support alternative formats while still supporting existing queues and handlers.

[stale]

This issue has been automatically marked as stale because it has not had any recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[martincostello]

Looks like this overlaps with #1003.