chore(handlers): define counters interface to store and use data

Context

This is related to Download counter for container images (&14021) and proposed implementation plan.

Problem

The registry needs to store the number of times a container repository has been downloaded.

Proposal

The GetManifest handler needs to define an interface to save each download count for both HEAD/GET requests.

• The interface should describe the operation of upserting a counter in the counter/statistics store.
• Implement an in-memory store to enable this functionality without requiring Redis (see #1301)