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>

pkg/models/package_type.go

@@ -16,6 +16,7 @@ const (
 	PackagePuppet       PackageType = "puppet"
 	PackageTerraform    PackageType = "terraform"
 	PackageGoProxy      PackageType = "goproxy"
+	PackageCargo        PackageType = "cargo"
 	PackageGitHubRPM    PackageType = "github_rpm"
 	PackageGitHubDeb    PackageType = "github_deb"
 	PackageGitHubAlpine PackageType = "github_alpine"
@@ -33,6 +34,7 @@ var validPackageTypes = map[PackageType]bool{
 	PackagePuppet:       true,
 	PackageTerraform:    true,
 	PackageGoProxy:      true,
+	PackageCargo:        true,
 	PackageGitHubRPM:    true,
 	PackageGitHubDeb:    true,
 	PackageGitHubAlpine: true,

pkg/models/package_type_test.go

@@ -18,6 +18,7 @@ func TestPackageTypeValid(t *testing.T) {
 		models.PackagePuppet,
 		models.PackageTerraform,
 		models.PackageGoProxy,
+		models.PackageCargo,
 		models.PackageGitHubRPM,
 		models.PackageGitHubDeb,
 		models.PackageGitHubAlpine,