Fetch on-boarded Package Descriptors

Originated from [#163](https://git.cs.upb.de/5gtango/UserStories/issues/163). Will also cover search-related issues if implemented with URL parameters, like in `...?field1=value1&field2=value2&...`. This API [exists](https://github.com/sonata-nfv/son-gkeeper/wiki/Packages-usage#get) but needs to be adapted.