Convert class ScopeVariantsForSearch to a service class

[luisramos0]

What we should change and why (this is tech debt)

We should convert ScopeVariantsForSearch into a simple service class called something like VariantsSearchService. This class is not "scoping" variants to hubs as ScopeVariantToHub but rather doing a very normal search with filters on variants and, in the process, it also scopes the variants using ScopeVariantsToHub. It just a matter of a bad name for the class that is very misleading. This code should also be moved from /lib to app/services.
This should be done so the code becomes better organized and simpler.
I am not sure there is a tech debt issue for it but as we do this we should think about how to remove subscritptions logic from the core search functionality in OFN.

Context

This came up in #3172

Impact and timeline

This is not critical but because it's a central piece of the OFN codebase it deserves some attention. As OFN and its API grows, catalog search will become more important.