Add support for using Vitess primary key as Kafka message key

Vitess recently added primary key information in the `FIELD` event. This means we know which fields are the PK fields. We need to use this information to set the SourceRecord key.

Related vitess issue.

A Vitess table's primary key will be used as the Kafka message key, or the unique key if the table does not have a primary key.

We will not support multi-column keys because the flags provided by VStream doesn't have sufficient information for multi-column keys.