title: KSQL Kafka formatting of keys
body: <p>One of my topics has a string-json as a key - {"city":"X","id":22}.
  In my ksql statement I want to extract it to 2 different fields not one so I can later filter and join.  In the docs it seems to allow me to stick only the whole string into the key instead of allowing to format it as JSON (just like FORMAT_VALUE) see below... Any suggestions for an elegant solution to this?</p>
  
  <p>VALUE_FORMAT (required) Specifies the serialization format of the message value in the topic. Supported formats: JSON, DELIMITED, and AVRO</p>
  
  <p>KEY Associates the message key in the Kafka topic with a column in the KSQL stream.</p>