add support for surfacing heartbeat messages in topic subscription

[cprice404]

We had an inquiry from a user about how to observe the health of a topic subscription. They expressed interest in getting access to the heartbeat messages; this would be exposed as either a new flag on the subscribe API, or an alternate subscribeToAllMessages API. When the user opted in to receiving the more advanced message types on the subscription stream, then those would be surfaced as they arrive.