JsonSerde only handles JSON objects, not primitive types

[moritastic]

For example, JsonSerde should be able to handle a Kafka record key that is just an (string encoded) integer. We could handle this by having primitive specific serdes, but they'll need to be able to convert these primitive types into the expected BaseRecord class. And having JsonSerde handle this out of the box simplifies usage for text based representations.

Another potential issue that should be solved here, is we should handle types like BigInteger in the ByteArray class.