EntityDeriverBase should inject the string translation service

EntityDeriverBase uses StringTranslationTrait which says this in it's docblock:

* If the class is capable of injecting services from the container, it should
* inject the 'string_translation' service and assign it to
* $this->stringTranslation.

So, we should do that! My patch on #2561935: Create a block for every field (which'll take the entity as context) injected it, so if the new deriver on that patch is going to use EntityDeriverBase, I'd like it to continue to be injected. :-)