This task is for determining basic stream configuration syntax. We can collect requirements and bikeshedding here to the extent that we need to, so we don't spread the discussion out over too many other tasks. Will update description with ideas and proposals.
The an example of thee current syntax is documented in the README file for the EventStreamConfig extension:
In use syntax is in InitialiseSettings.php in mediawiki-config.
Design document focusing on producer syntax can be found here.