Sparse Registry Support

[onelson]

A goal for cargo is to stabilize "sparse registry" support.

Adding support for this would (very roughly) mean Estuary would need to expose the index over HTTP instead of, or in addition to, the git-based index it offers today.

Ref:

• Stabilize sparse-registry rust-lang/cargo#11224
• Tracking Issue for serving an index over HTTP rust-lang/cargo#9069
• sparse registry: determine UX and URL handling rust-lang/cargo#10964
• https://github.com/rust-lang/cargo/blob/c015bfd1efb04c0bf60df9bfef2b8b3b6633310b/src/doc/src/reference/registry-index.md#index-protocols