Registers the request/response MQTT topic pair for commands on the specified entity. Accepts either the legacy single-command form or the bulk map form on one URL; the response mirrors whichever form arrived (the flat topic object for the single form, the results envelope for the bulk form). Registering N commands one at a time costs N requests; the bulk form costs one. Existing topics for a command name are updated, so the call is idempotent and safe to retry. In the bulk form, either every topic pair is written or none is.