changefeed: add protobuf as row emission format

[sheaffej]

Epic: CRDB-9086

Customer requests to add protobuf row emission format for Changefeeds, for example when emitting to Kafka.

https://www.cockroachlabs.com/docs/stable/create-changefeed.html

The CREATE CHANGEFEED statement creates a new enterprise changefeed, which targets an allowlist of tables, called "watched rows". Every change to a watched row is emitted as a record in a configurable format (JSON or Avro) to a configurable sink (Kafka or a cloud storage sink). You can create, pause, resume, or cancel an enterprise changefeed.

Customer requests consideration of adding Protobuf as a row emission format to Kafka because they use Protobufs for all messages between systems in the their organization. They can work around this and use Avro by writing an Avro-to-Protobuf converter, however, this will be an outlier piece of code that will be awkward to maintain as they have standardized on Protobuf as the data exchange format.

Jira issue: CRDB-9088

[amruss]

Putting on the backlog for now and will revisit if there is a strong customer need - Would be tricky due to schema registry edginess.

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