Add project count for namespace API when admin

What does this MR do and why?

We noticed that zoekt node memory can struggle when loading several shards at once. Adding a namespace with an extreme amount of projects saturated the zoekt webserver.

This adds projects_count to namespace API for admins. This will be used in /chatops run namespace find <namespace_id> to help with zoekt rollout. We can prioritize adding customers with fewer projects as they are more lightweight.

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