Refactor callback mechanism

I've added onInit, onAccept and onDecline options to the Klaro App entity and I've updated the form in MR!79

Tested MR!79 working. Generated the patch file.

Updated status, as JS logics are missing.

Updated patch. Passed onInit, onAccept, onDecline to services' config settings.

Rebased and rerolled the patch.

Big thanks to kensae and ckng. I have to check in the next days and will add some tests for the new options.

I think the update for existing Klaro app entities should also be provided, since the entities now have new properties and the existing configurations are invalid for the new schema. I'm curious about whether this update should be more cautious, considering how long it has been since this issue was created. In its current state, it will overwrite all existing callback data after the update, which could be a bit intrusive. Perhaps only set a default value if no value exists at the time of the update? This way, projects that relied on this merge request until the official release will retain their configuration.

I think we cannot write an update hook for old javascript snippets. We have to use both variants in parallel and deprecate the old way in next major release.

Yes, we can't update the existing JS snippet. But, there are other problems related to the new properties:

1. Shouldn't we provide a proper update for klaro_app entities in order to set default values for newly created properties? I have tested this locally and it looks like Drupal automatically detects this and, on the next configuration export, adds missing properties.
2. Shouldn't we update the default klaro_app entities (./config/install) to match the schema? Drupal seems to do this at some point, by merging missing properties. But currently, they are becoming out of sync with actual entities, and this might be a problem in certain scenarios.
3. The description of on_decline should be checked and updated to make it more specific when the callback is called. At the moment, I'm unsure which is correct, but it seems that the behavior is correct, so the description needs to be updated.

Thank you, niklan! You are of course absolutely right that we need an update hook for the addition of config entities and that the schema also needs to be updated. (I had understood that we would split the existing scripts into the new areas - which I think is too risky.)

I played around with the events and observed the following sequences, starting without the klaro cookie:

• A: Load: onInit, onDecline, deprecated User declines: onDecline, deprecated
• B: Load: onInit, onDecline, deprecated User accepts: onAccept, deprecated

Subsequent page loads:
Load: onInit, onDecline|onAccept, deprecated

For me it looks like the behavior is sound, as niklan mentioned in #15.3
Applying this sequence for googles consent mode, one could use the onInit event to send the default consent state (denied) and then use the onAccept event to communicate a possible consent.

Regarding the description one could write, that the consent was explicit or also implicit, e.g.:
This javascript will be executed after the user has explicitly or implicitly (page load) declined the service.

When it comes to the update script. The cautious approach would be to set the properties only, when not present. Something like:
$klaro_app->set('on_init', $klaro_app->get('on_init') ?: '');
right?

Regarding #15.2: For me this looks like a lot of work as all services would need to be migrated and probably tested. As far as I understand, they would just work like before. Why not adapt them one-by-one afterwards when needed, with people involved, who know these services well and can test them?

Regarding #15.2: For me this looks like a lot of work as all services would need to be migrated and probably tested.

In 15.2, I've talked only about entities in `./config/install`, which are used for new installations (when the module is installed, like `hook_install()`). Changes here won't affect existing installations. However, it looks like Drupal provides missing data from default values, so it is mitigated in a way.

Basically, they are pre-exported entities, which are simply outdated from what they actually will be, but since the user will export configuration changes after a new module install regardless, they will be valid ones in the configuration sync directory.

This javascript will be executed after the user has explicitly or implicitly (page load) declined the service.

This one is much better, because it doesn't imply that user have to do any action for this event to occur.