Add migrate process form plugins

I think that improvements can be made to the provided process plugins:

1. A typo in EntityGenerateForm: 'ke2:value2'
2. EntityLookupForm could let the user choose entity type and bundle from a list - and I think autoset bundle key this way?
3. For some string translations double quotes are used where single quotes could be used as well.
4. In some cases the field descriptions are nearly the same as the labels, like in EntityLookupForm:
   

   $form['bundle_key'] = [
     '#type' => 'textfield',
     '#title' => $this->t('Entity type bundle key'),
     '#description' => $this->t("The entity type bundle key."),
     '#default_value' => $this->configuration['bundle_key'],
   ];
   

   In this case either omit the description or provide a more useful description.

5. Maybe it would be good idea to add functional tests for each process plugin that we add a form plugin for.