inter-suite triggering via message broker (built-in).

[hjoliver]

@TomekTrzeciak has argued (see this off-topic conversation on another issue: #2749 (comment)) that is would be good to have built-in support for decoupled inter-suite triggering (i.e. without the current tightly coupled suite-DB polling; and without relying on an xtrigger call to an external system like Kafka).