Start moving code from the ECA module

I am adding testing instructions to the issue summary.

On second thought, I do not think we need to move the Modellers service to the modeler_api module. It consists of convenience methods (mostly wrapping the plugin manager) and some methods closely tied to the eca module (such as events(), which returns a sorted list of all the Event plugins). We may decide to move some of that to the modeler_api module, but not as part of this issue.

Other than the service, I think the MR handles everything in the issue description. This issue is ready for review, along with the related issues for the eca and bpmn_io modules.

Great work Benji

Additional thoughts:

* Talked with Jurgen this morning and discovered that there may be a strong need to move the existing BPMNModeler bits that are in the eca module into the BMPN.io module.
* So we can see what the moduler_api needs to know in order for that to work.

> ... there may be a strong need to move the existing BPMNModeler bits that are in the eca module into the BMPN.io module.

I already added 🌱 Decide what to do with the modeller_bpmn module Active to work on that question.

As discussed in Atlanta, I'm spending 3 full days on building the initial structure on top of what @benjifisher and @cosmicdreams have already built. For that here are some names, that I'm using for now:

• Model: config entity, owned by the ModelOwner
• ModelOwner (a.k.a. client or target): plugin like ECA
• Modeler: plugin like bpmn_io

And I've the following changes as well:

• ECA has no dependency on modeler API or bpmn_io
• bpmn_io has no dependency on ECA
• PHP 8.3 or later required
• Drupal core 11.2 or later required
• There will be no modeler config entity any longer, the raw data will be stored in the model itself, performance considerations addressed later

My first commit contains all those changes and the plugin manager and interface for ModelOwner is included and contains most of the skeleton code.

I'm now working on the plugin manager and interface for modelers, then I can get the whole thing working hopefully by tomorrow.

All tests are now green and this is ready to be merged as a baseline to be followed-up in separate issues from here on.