Add option to disable ECA programmatically

We already have a kill switch for ECA, which is implemented in \Drupal\eca\EventSubscriber\DynamicSubscriber::onEvent. Would it be possible to move the suggested kill-switch from this MR into that context as well? That way, we have the same purpose combined in the same location, certainly better for long term maintainability.

Yeah, this might work too. Or move the logic from the event into the Processor. DynamicSubscriber should be as "dump" as possible and only serve one purpose. Feels a bit odd to disable the processor there.

The intention back then has been to provide a kill switch that disables all of ECA as early as possible. And that's in the subscriber, where it comes with no overhead except for that 1 line to check if the switch is set or not.