Allow ingestion from multiple registries

Problem/Motivation

The current approach assumes a single registry associated to a Drupal site. This limitation is artificial an the result of lack of anticipation. We should ingestion from multiple endpoints.

While we are at it, we should declare each registry that we ingest from as a configuration entity. This will allow sites to extend functionality to apply business logic on different registries (via 3rd party properties and custom logic).

This will hopefully allow to have public registries with generic components, and private ones.

Proposed resolution

Introduce a new configuration entity type to represent different registries that can contribute widgets to the site. Each widget will then have access to the registry its type was ingested from. With that, more advanced and nuanced logic can be applied. For instance: require more restricted permissions to edit widget instances for RegistryA, but not RegistryB.