Support the mapping additionalProperty with properties_field

It might be worth it to support https://www.drupal.org/project/physical → first.

@jrockowitz No. physical module can only support dimension and measurement properties, not arbitrary. I test several modules before posting here.

@jrockowitz I today find there is "Map" subfield type of the similar feature of arbitrary inputting key-value paris in the custom_field, which is already supported by schemadotorg. So if the JSON+LD and other mappings with the "Map" subfield of custom_field can be better working, it would be wonderful!

I really like the custom_field module, and adding support for the 'Map' data type is definitely on the roadmap.