roachtest: add order validation to CDC Kafka roachtests

This patch adds order validation to CDC Kafka roachtests so that we
can build more confidence in our ordering guarantees. It can be enabled
for a roachtest either by directly setting the `validateOrder` flag on a
`kafkaManager` before creating consumers, or indirectly by setting the
`validateOrder` flag on `kafkaFeedArgs` for tests that use `cdcTester`.

Release note: None