Add process plugin to generate entity translations

Problem/Motivation

As far as I know, there's no way to generate entity translations when migrating multilingual content. entity_generate only creates new entities, not translations. The current state of #3227245: Entity lookup doesn't work with multiligual content → doesn't add the possibility to create translations, only load pre-existing ones.

Proposed resolution

Create a new entity_translation_generate plugin extending the entity_lookup plugin. This plugin wil share the same configuration as entity_lookup, just like entity_generate. Additionally, a translation_langcode option should be provided to tell the plugin in which language the entity should be translated. We could also add the option to get the langcode from default_values or values if it's dynamic. The entity can only be translated if the entity_lookup plugin is able to load the original.