Add cargo sparse registry remote type (#132)

Rust crate builds (rpmbuilder CI) fetch from crates.io directly because artifactapi has no Cargo remote type.

- add `cargo` package type proxying the sparse registry protocol (RFC 2789)
- synthesize `config.json` so `dl` points crate downloads back at the remote
- treat index files as mutable and `crates/*/*.crate` as immutable, fetched from static.crates.io for crates.io
- add a `~/.cargo/config.toml` usage snippet to the UI and a cargo e2e caching case

Reviewed-on: #132
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

ui/src/pages/Remotes.tsx

@@ -17,6 +17,7 @@ const typeColors: Record<string, 'blue' | 'green' | 'yellow' | 'red' | 'default'
   puppet: 'yellow',
   terraform: 'blue',
   goproxy: 'green',
+  cargo: 'yellow',
 };
 
 export function Remotes() {