Add nuget v3 remote type (#141)

.NET builds restore from nuget.org directly because artifactapi has no NuGet remote type.

- add `nuget` package type proxying a NuGet v3 feed; `index.json` maps to the upstream service index
- add a `ServeRewriter` provider hook so cached JSON has upstream-origin URLs pointed back at the requesting host per request
- treat `.nupkg`/`.nuspec` as immutable and service, registration and flat container indexes as mutable
- add a `nuget.config` usage snippet to the UI and a nuget e2e caching case

Reviewed-on: #141
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

ui/src/components/UsageInstructions.tsx

@@ -243,6 +243,22 @@ index = "sparse+${proxy}/"`,
         },
       ];
 
+    case 'nuget':
+      return [
+        {
+          title: 'Use this feed instead of nuget.org',
+          language: 'xml',
+          code: `<!-- nuget.config -->
+<configuration>
+  <packageSources>
+    <clear />
+    <add key="${name}" value="${proxy}/index.json" protocolVersion="3" />
+  </packageSources>
+</configuration>`,
+          note: 'Serve artifactapi over HTTPS: NuGet rejects the repository signatures resource over plain HTTP.',
+        },
+      ];
+
     case 'puppet':
       return [
         {