Support for sparce registry

Hi. I want to ask if you have any plans to support [sparse registries](https://blog.rust-lang.org/2022/06/22/sparse-registry-testing.html). This could be especially helpful with mirroring, since crates.io index is very heavy.

Or if you could give some tips on how this could be done and where to start I could try to implement this (although I never used Clojure). 