Add batch fetching of media

UX interacting with the medialibray could definitively use a system to fetch multiple media from a list of unique identifiers. For now, the current use cases identified by the VLC desktop team are only fetching media via a list of ID or MRL.

The API calls would probably look like this:

virtual Query<IMedia> media( std::set<int64_t> identifiers, const QueryParameters* params = nullptr ) const = 0;
virtual Query<IMedia> media( std::set<std::string> uris, const QueryParameters* params = nullptr ) const = 0;

The final use-case is to fetch by MRL all the media potentially indexed in the service discovery view in a single request.