Extract entity catalog from content pack specific code

## Description

The `/system/catalog` endpoint provides a catalog of all entities in the system. The code for this is currently embedded in the content pack specific code. Since the catalog will be used for features unrelated to content packs, we want to extract the code into its own package so we can use and extend it independently from content packs.

Our first use case will be the resolving of GRNs into entity descriptors (e.g. title, description, database ID) to allow the UI to show details for a GRN.

## Tasks

- [ ] tbd