CloudEvents: Propagate "partitionkey" [DBZ-4818]

Migrated from [DBZ-4818](https://issues.redhat.com/browse/DBZ-4818)

As per the CE [partitioning extension|https://github.com/cloudevents/spec/blob/v1.0.1/extensions/partitioning.md], the "partitionkey" attribute "can be used to determine which bucket each event goes into". We should set it to a value derived from the change event key. I can think of two ways:
* Stringified JSON-representation of the key, e.g. {{"\{\"id\" : 1234, \"sub-id\" : 56}"}}
* semicolon-separated concatention of all key field(s), e.g. {{{}"id:1234;sub-id:56"{}}}, similar to what we do for the "id" field

I'd vote for the latter. Any thoughts?

The issue names no repository files, tests, or entry points. Start with the linked CloudEvents partitioning extension and locate the code that maps Debezium change-event keys to CloudEvents attributes. Done requires an agreed partitionkey representation and verification for the proposed key formats.

Written by the indexing model from the issue text.