Add a topic name configuration for the heartbeat messages

Currently the heartbeat uses the following format topic name to send the empty messages '<heartbeat.topics.prefix>.<server.name>'. As a result, this will require us to create topic for each source which could be overkill if you are running high number of sources, especially if these data emitted are not important, hence having only one topic name for the heartbeat configured for all sources would be more appealing.
We can help on this one and send a PR