Add search endpoint to find Listings by feature ids
Created by: creckord
In the Marketplace Client, we face the challenge that we might have a pre-existing installation, and/or installs happening through other means than MPC, yet still want to offer the usual MPC support for the installed items, if they are also available in the Marketplace.
In order to do that, we need to identify the Marketplace listings based on the installed features. In the old MPC, this was done on a best-effort basis memorizing feature_id <-> Listing mappings for all listings that MPC happened to come across. This mapping was usually woefully incomplete.
In the new API, it would therefore be great to have an endpoint that takes a list of feature ids and returns all Listings that contain at least one of these ids in their effective Solution versions.
I'm not sure if the existing GET /listings endpoint is a good fit, simply because the list of feature ids can get somewhat long. I imagine a POST endpoint would be a better candidate in this case.