Let source plugins inject tokens to pushed content

Thanks @Balu Ertl for your contribution, this is great to see.

The approach in the MR is problematic because DANSE is not a dependency for the push framework. Instead, this token should be provided by the DANSE module.

BTW, we are integrating both modules (DANSE and Push Framework) with ECA. That way, you could handle all such cases there without having to have any custom code. See #3325561: Integrate with ECA. Just saying, we could still also add some hooks as suggested but maybe that no longer required then, because we would have events available to which you could subscribe.

I've now implemented a different approach for this: when the SourceItem calls prepareContent from the channel plugin, it forwards the source plugin and the source id, and while preparing the content, those two arguments are forwarded as token data called push_framework_source_plugin and push_framework_source_id to the token replace function.

This way, we are completely generic, regardless from where the data is coming that's being pushed.

The source of the push notification, in your case DANSE, can then implement some tokens that will be able to load the notification by using the provided data and use that to replace them. This will be addressed in #3328626: Provide tokens for notifications.

Changing title.

Cool stuff.

Concerning the code comment convention, I propose to do this all in the cleanup ticket. Like we did in ECA.

Excellent