Create KT for ksqlDB and nested JSON object with non-uniform fields

When working with nested JSON in ksqlDB, a standard approach is to use the `STRUCT` keyword. But if the nested objects don't have uniform fields, then another approach using the `MAP` function is required.

Community ref [from stackoverflow](https://stackoverflow.com/questions/64241285/kafka-topic-with-variable-nested-json-object-as-ksql-db-stream).

@rmoff has a [blog post](https://rmoff.net/2020/10/07/ksqldb-how-to-model-a-variable-number-of-fields-in-a-nested-value-struct/) detailing the solution, but this would be a great addition to KT.