Maven VReg: utilize CounterAttribute concern to increment cached responses downloads count

Context

The downloads_count column in the virtual_registries_packages_maven_cached_responses database table needs to be incremented whenever a cached response is downloaded. In #473152 (comment 2117647182), it was agreed on using the CounterAttribute concern to implement the functionality.

Also, have a downloaded_at column with the last timestamp. This can quite easily done due to the fact that the CounterAttribute concern use update_counters. The rails helper function has us covered as it has a :touch option where we can pass a (timestamp) column that will be updated along with the counter. Thus, we can override the #update_counters (instance function) in the cached response model, so that we pass touch: :downloaded_at to the class function.

⚙️ Technical details

• We need to have downloads_count and downloads_at on the cache entry record.
• Use the CounterAttribute to implemented a delayed count update. This way, during the GET request, these fields are not updated = the entire request will use the replica (read only).