cdc: add param for schema registry to specify the subject name

When creating a changefeed that uses Schema Registry and a specific topic, CRDB queries for the key schema using the _table name_ as the `subject` instead of the `topic_name` specified in the changefeed.

```
create changefeed for table table1
into
'kafka://kfk:9092?topic_name=topic1'
with
format = avro,
confluent_schema_registry = 'https://sr:8081/';
```

This will result in a HTTP POST call to this endpoint:

```
Post: 'https://sr:8081/subjects/table1-key/versions'
```

instead of

```
Post: 'https://sr:8081/subjects/topic1-key/versions'
```

This request is to be able to pass a parameter to specify the Schema Registry subject name independently of topic or table name.

[Slack](https://cockroachlabs.slack.com/archives/C9TGBJB44/p1701898319420529)

Jira issue: CRDB-34206

gz#19626

Epic CRDB-25039