Connect Transforms support for nested structures

Single Message Transforms (SMT), KIP-66, have greatly improved Connector's usability by enabling processing input/output data without the need for additional streaming applications. These benefits have been limited as most SMT implementations are limited to fields available on the root structure:

• https://issues.apache.org/jira/browse/KAFKA-7624
• https://issues.apache.org/jira/browse/KAFKA-10640

Therefore, this KIP is aimed to include support for nested structures on the existing SMTs — where this make sense —, and to include the abstractions to reuse this in future SMTs.

KIP: https://cwiki.apache.org/confluence/x/BafkCw