Add gems remote type for RubyGems (#142)

Ruby builds fetch gems from rubygems.org directly because artifactapi has no RubyGems remote type.

- add `gems` package type proxying `https://rubygems.org`-style sources
- treat `versions`, `names`, `info/*` and `*specs.4.8.gz` as mutable, `gems/*.gem` and `quick/Marshal.4.8/*.gemspec.rz` as immutable
- serve `api/v1/*.json` as `application/json`
- add a gem/bundler usage snippet to the UI and a gems e2e caching case

Requires unkin/artifactapi#148 (HEAD route; RubyGems probes `HEAD /versions`).

Reviewed-on: #142
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

pkg/models/package_type.go

@@ -17,6 +17,7 @@ const (
 	PackageTerraform    PackageType = "terraform"
 	PackageGoProxy      PackageType = "goproxy"
 	PackageCargo        PackageType = "cargo"
+	PackageGems         PackageType = "gems"
 	PackageNuGet        PackageType = "nuget"
 	PackageGitHubRPM    PackageType = "github_rpm"
 	PackageGitHubDeb    PackageType = "github_deb"
@@ -36,6 +37,7 @@ var validPackageTypes = map[PackageType]bool{
 	PackageTerraform:    true,
 	PackageGoProxy:      true,
 	PackageCargo:        true,
+	PackageGems:         true,
 	PackageNuGet:        true,
 	PackageGitHubRPM:    true,
 	PackageGitHubDeb:    true,

pkg/models/package_type_test.go

@@ -19,6 +19,7 @@ func TestPackageTypeValid(t *testing.T) {
 		models.PackageTerraform,
 		models.PackageGoProxy,
 		models.PackageCargo,
+		models.PackageGems,
 		models.PackageNuGet,
 		models.PackageGitHubRPM,
 		models.PackageGitHubDeb,