services: added record extension registry

[alejandromumo]

closes #1878

---

This PR introduces a new mechanism to extend the configured record class.

The idea is that some modules might want to add a single field (or method) to the record class, without polluting the core's RDM namespace.

The interface of these "extensions" is a callback that returns a valid Record class.

The extension point right now uses entry points since they can be loaded before all the extensions are initialised.

One example of a registration could be the following:

class SWHExtension(object):
    swhid = SystemField("swhid")


def add_swh_extension(app, record_cls):
    # generate a new class with the field and return it
    if current_swh_ext.is_enabled(app) and current_swh_ext.is_configured(app):
        return type(record_cls.__name__, (record_cls, SWHExtension), {})
    return record_cls

There are still unanswered questions to move forward:

• The field can't be stored in DB or ES, if both mappings are set to strict
• Dumpers are not extensible at the moment
• Serializer's schemas are not extensible from config, instead, they are extensible on object initialisation. Therefore, we can't extend it from other modules than invenio-rdm-records (without subclassing the serializer entirely)

[alejandromumo]

TODO: to be removed

[alejandromumo]

This is the place where the interface is being enforced

[github-actions]

This PR was automatically marked as stale.