Action "List: add item" ignores specified key when setting a numeric key

This is a challenging one. Should we probably introduce a flag in \Drupal\eca\Plugin\DataType\DataTransferObject to optionally turn off rekeying which should be called right before calling $list->set($index, $item); in the action plugin?

Should we probably introduce a flag in \Drupal\eca\Plugin\DataType\DataTransferObject to optionally turn off rekeying which should be called right before calling $list->set($index, $item); in the action plugin?

Either this, or initialize a DTO with a rekey option, or do both. Maybe then let the user decide whether rekey should happen, by adding a corresponding option to the "List: add item" action? There is another place where someone can add an item to a list, for example with "Token: set value" using token name [mylist:+]. But I think that one is probably never used by anyone. Still there may be other places where an item is being added to a list (other actions, such as merging).