cdc: support composite types in avro

[HonoreDB]

User-defined composite (tuple) types will be released and usable as table columns in 23.1. Currently a changefeed in avro format will not be able to serialize them, but it should be straightforward to represent them in avro as a nested record.

Jira issue: CRDB-27718
Epic CRDB-27835

[blathers-crl]

cc @cockroachdb/cdc

[katmayb]

cockroachdb/docs#16972

[XiaochenCui]

Hi, @HonoreDB :

Has this issue been resolved? If it still exists, could I please get permission to access JIRA to take a closer look?

Thank you!