Make all token data providers discoverable and expose them in the UI and in the ECA Guide

As we get more and more integrations into ECA that come with Events that certainly can't implement the DataProviderInterface, we should step away from this suggestion and instead turn ECA event plugins or EventSubscribers into data providers. This would make this approach consistent across all events because we have both for all events, whether they come from ECA, Drupal core or other modules.

That said, token declaration should be moved away from Events.

If there is no objection, I'm going to start working on this, also to expose all such provided tokens to the UI - and to the ECA Guide.

As we came up on this in ✨ Performance optimization: dynamic event subscription Fixed we concluded that it most probably makes sense to make the event plugins the proper place for providing token data.

Could also move any ECA-related interface logic into the event plugin, such as ConditionalApplianceInterface. Could also be addressed as another follow-up then. Benefit would be that system events can then be left untouched from ECA-specific things.

Possible blocker (or not): 📌 Enable event plugins to determine appliance Fixed

I've updated the issue summary and added this together with 📌 Enable event plugins to determine appliance Fixed to the change record → for later write-up of what other maintainers will have to adjust in their integrations.

MR 408 now contains a WIP for tokens discovered from PHP attributes on a couple of methods in the ECA event plugin.

What's missing and needs some more thoughts:

• Potential tokens provided by parent classes
• Tokens provided by event subscribers

This is working very nicely already. Only a couple of related issues are left open, but the basic functionality is in place and already published on the ECA Guide.

The tokens are now exposed in the template tab of the bpmn_io modeller, until we get support for a better place and formatting from upstream packages.

Automatically closed - issue fixed for 2 weeks with no activity.