Allow additional conditions in entity_lookup

Thanks for the patch, i can confirm it works !

The entity lookup process plugin was written to be extended. Adding a custom process plugin is relatively easy. While this is a pretty well tested and easy feature to add to the module, I want to resist adding any new functionality to entity lookup.

Rerolled against the latest 6.0.x

Thanks @idebr, this would be a nice feature to have in on of my favourite Migrate Plus plugins, entity_lookup :)

I have added an example in the "Proposed resolution" section.