Add Sink connector format option

[jelledv]

Add option to add a formatter when writing data to RabbitMQ:

• Added the sink connector option "rabbitmq.format"
• Before you had to use the ByteArrayConverter to use the RabbitMQ sink connector. Now you can use a formatter to put another format on your queue(s). Currently an implementation has been written for json and (non confluent) avro.
• This change is backwards compatible, when no formatter is provided, the default value "bytes" is used. This requires you to use the "org.apache.kafka.connect.converters.ByteArrayConverter" converter like before