Add field to be able to record VG Wort participant information and new plugin API to extract this from entities

Problem/Motivation

As documented in https://tom.vgwort.de/Documents/pdfs/dokumentation/metis/DOC_Verlagsmeld... call the new message API to register content from a site. In order to do this we need to build a VG Wort participant list for an entity.

Proposed resolution

• Add a new vgwort_participant_info field that can store the necessary details to create a list of participants based on this field.
• Add a new plugin manager that can use plugins to build a list. This abstraction allows use to merge participation information from multiple fields on an entity and also allows us to provide a plugin to get the same information from a user entity that is an entity owner.

Remaining tasks

User interface changes

New field and field widget. The field formatter is rudimentary as it is unlikely to be needed.

API changes

• New field plugins
• New VG Wort participant list plugin manager
• New VG Wort participant list plugins

Data model changes

New field data.