Augment dynamic beans with plugin metadata: Taxonomy use case

A document with one or more taxonomy fields will contain a string (multiple) property where the selected keys are stored. These are automatically exposed in dynamic beans since it's a simple string property.

This issue is about exposing more content than just the keys via the dynamic beans generation. For taxonomy specifically, we'd like to expose the labels of those keys: this content exists in the taxonomy document from which the keys were selected. The definition of the field in the document type contains the path to that taxonomy document.