Move uriSchemes to *StoreConfig

It is a property of the configuration of a store --- how a store URL is
parsed into a store config, not a store itself.

Progress towards #10766

src/libstore/local-binary-cache-store.hh

@@ -15,6 +15,8 @@ struct LocalBinaryCacheStoreConfig : virtual BinaryCacheStoreConfig
         return "Local Binary Cache Store";
     }
 
+    static std::set<std::string> uriSchemes();
+
     std::string doc() override;
 };