Add support for generic message keys (e.g. message keys in Avro or JSON format, STRUCT)

[apurvam]

Right now, KSQL only uses a string Serde for keys in kafka messages. This works well for doing joins and aggregates on the entire key. However, in some cases a key (in json or avro or any other format) may have nested fields, and we may want to do joins and aggregates on a per field basis.

Further, by supporting structured keys, we can do filters on the fields of the keys as well.

This is related to #638 and #804.

We should also support non-string keys, such as Integer.

[mquraishi]

Additionally, 1) If the value (not key) is an avro containtered object with the schema visible in clear text but the payload itself is a bytearray, KSQL cannot deserialize correctly. In code it is easy. This is once the message is retrieved from the topic. 2) If the value is json and one of the elements is another nested json object, creating a stream turns the nested son into some dictionary key=value object, resulting in deserialization errors.

[miguno]

For the record: Another +1 from a user (offline conversation).