[4.x] - Entity reference producer with a single value?

Hello!

I have a case where I have an entity with a reference to a media entity, which has a reference to an image entity which I would like to produce the URL for.

I expected to be able to get the referenced entity by just chaining `entity_reference` producers, something like this:
```
$builder->compose(
$builder->produce('entity_reference')
->map('entity', $builder->fromParent())
->map('field', $builder->fromValue('field_image')),
$builder->produce('entity_reference')
->map('entity', $builder->fromParent())
->map('field', $builder->fromValue('field_media_image')),
$builder->produce('image_url')
->map('entity', $builder->fromParent())
)
);
```
The problem I'm facing is that the first `entity_reference` producer always returns an array, even though I know in this case it's a singleton item, and that doesn't play nicely with the second `entity_reference` producer call. Could it be possible to map in a context value that tells the producer to only return the first item in the array? It would make use cases like this possible.

For now, I solved this by chaining `property_path` and `entity_load` for each reference but it feels like it's very verbose in comparison to the above example.