Binary Payloads

[iancooper]

There are some issues around binary payloads, such as those expressed in #2458

This PR is about the following:

• Move Kafka to use a byte[] payload not a UTF-8 string as this will corrupt Kafka header bytes on the payload when used for schema id
• Add an explicit character encoding to the body, so that we know the character encoding of the string supplied, or can ask that any string created is in a format. Add a helper method to display in a different format
• Fix how we write to DynamoDb, which needs a Base64 string when storing binary data and not the UTF8 string we give it.