Endpoint for listing all published packages in a Sink

[leftieFriele]

Following in the same line of thought as #581

Implementing an API endpoint where you can get a list of all available published packages. I know it's been requested many times to have a "UI on top of Eik".

This would be something that the Sink would have to provide I assume, but it would be really useful to have. Both for people new to Eik, but also for package discovery on the assets server.
Even if all packages are often available in a package manager too, this is a often requested feature.