Allow multiple templates using nuxeo-data-table for the same field type

The problem is easier to understand through an example:

Given that the Designer registries define these templates for the following field types:

• string
  • Text: using nuxeo-input (default template for edit mode)
  • Textarea: using nuxeo-textarea
• string[]
  • Text: using nuxeo-data-table (default template for edit mode)
  • Textarea: using nuxeo-data-table

Currently, when a string[] field is scaffolded using a nuxeo-data-table, the default view or edit template for string will be used on each of the rows of the table. This means that both templates for string[] will generate the same config, as they do not specify which element/template is supposed to be used inside it.
The goal of this ticket is to provide some flexibility to this mechanism, and allow specifying the element/template to be used directly from the registries.

Proposal: Reference the (single-valued) widget to be used by name.

"string[]": [
  {
    "default": "edit",
    "name": "Text",
    "is": "nuxeo-data-table",
    "rowWidget": "Text",
    "attributes": [...]
  }
]

This would represent the definition of a widget for a string[] property, using the nuxeo-data-table element. Notice how it references a "Text" row widget via rowWidget (could have another name). This "Text" widget should be a valid widget defined for the string field.