A new class is added which will add meta tags for dates and events.
The class has an interface which allows it to be replaced via DI to
alter behaviour.
Refactor import regarding data handler. We now also need to add a new
column "keywords". We use the new DataHandler approach.
But that approach only covered relations so far. We therefore refactor
that area to be more generic and use that one for new keywords column.
Relates: #10642
A new PSR-14 event is added that allows to modify the event right before
it is persisted within Destination Data One import.
This for example allows to alter dates, prices, etc.
Improve handling of missing end time in dates.
The clean up task had an issue to clean up to many files.
It didn't check if a file that could be deleted
because one event got deleted, was still in use by another event.
Relates: #10499
The existing code didn't work with the provided data.
We added a new test case to cover the situation.
We adjusted the existing code to check on the actual rel value instead
of the keys.
