migration_lookup process plugin can't use specific subset of source keys to use for lookup

I couldn't get #40 working. Here's an updated version that works on Drupal 10/11.

This also needs to be an MR at this point.

As per #47 - needs to go to an MR.

Removing

> 1. The ability to specify multiple source keys

from the IS and the title, as this is now possible:

        $lookup_value = array_values($row->getMultiple($this->configuration['source_ids'][$lookup_migration_id]));

Updated IS again.

There's a backwards compatibility & DX problem we haven't spotted.

We add support for this:

 *       source_ids:
 *         users:
 *           key3: author

but remove support for this, without keys:

 *       source_ids:
 *         users:
 *           author

That's a BC break, and it's also not good DX if you want to specify all the lookup keys.