Provide 6 UI port for MultiValueChildNodeTransformer

Original ticket description

Fields which use Nodes to store values will mutate Parent when creating a new child.

When creating a content app where you can nest nodes, you might run into following issue.

Given an item type whose form contains following multiValue Field:

- name: regions
  fieldType: multiValue
  label: Rehions
  transformerClass: info.magnolia.ui.form.field.transformer.multi.MultiValueChildNodeTransformer
  field:
    name: region  
    fieldType: select
    label: Region
    options:            
      - name: north
        value: north
        label: North  
    
      - name: east
        value: east
        label: East

      - name: west
        value: west
        label: West

      - name: south 
        value: south
        label: South

Magnolia 5 UI framework compatibility issue

Now in your content app you'd create an item which has north and west as values in the region field. Then you create a new item below aforementioned item. First you'd notice that the values off the parent item are already entered in the form for this new item. If you now remove those and replace the values for the region field with south and east and save the form, you can observe that the values in the parent item have been also changed accordingly!

I've traced it down to an issue with JcrNewNodeAdapter which is itself not an actual JcrItem so that composite fields and such would end up reading/writing values to the parent item.

As a workaround i've sublcassed JcrContentConnector and modified it so that it'll create an actuall Jcr Node if it's asked for a new Item Id. I've attached the code in this issue.

Magnolia 6 new UI Framework incompatibility issue

• Now in Magnolia 6 new UI Framework, we are missing an equivalent definition for MultiValueChildNodeTransformer. This makes customers having existing content cannot migrate to the new UI Framework. Content cannot be mapped and displayed properly anymore.

Workaround

• A workaround for the new UI Framework to have backward compatibility is expected as a temporary solution.