Track and store "download counts" from official registry

[tadasant]

Blocked on: https://github.com/modelcontextprotocol/registry/issues/94

Although a crude metric, it may be helpful to track "download counts" of every MCP server in the official registry. Only the official registry has insight into this data, and it can be used as a sorting signal by consumers.

If we do track this, it's worth being mindful of how this may negatively impact CDN caching - and worth designing around that.