Move pools metadata JSONs to separate folder

[refi93]

I get that we have the pools metadata in public due to historical reasons and it's tricky to switch because we'd have to update the pool registrations, but I'd consider at least moving in the codebase the metadata to public/pools_metadata subfolder so it's reasonably organized at least in the code, and serve them directly from our express backend, leaving there a clarifying comment why we are overriding the path when serving

related PR for context: #1190