System lookup table for entity metadata

## What?
A common requirement is to annotate certain messages with names of various entities, e.g. streams, inputs, forwarders, sidecars, and others.
Currently, there's no easy and unified way to get to that information, without going through the API using a token, but that is very expensive for something that is already available to the server that is running the processing.

This feature request is for creating a data adapter that exposes catalog-style metadata using multiple paths:
* translating a given ID and/or GRN (graylog's URNs) to the metadata
* looking things up by name/title

All entities available to the content pack catalog should be in this lookup table, as well as ephemeral things like running forwarders and sidecars, so users can annotate incoming messages.

## Why?
The message detail view shows you the forwarder and input names a message came in one, but there's no way to group on that field and decorate it with the human-readable name.
That could be done with a simple lookup decorator (although those are not supported in aggregations yet), or in pipelines with a custom field if the name should be searchable as well, but that information isn't currently available.

## Your Environment

* Graylog Version: Graylog 5.0+