Support multiple MQTT brokers

For me, it’s very annoying that it is still impossible to add multiple MQTT brokers.
Setting up bridges on my main broker for e.g my 4 openWB wallboxes, is really difficult to maintain and troubleshoot in case something screws up…

It could be solved with e.g a selection in the publish service and the MQTT trigger block, but with the option to add a “default” broker to not to break existing automations and scripts.