NuGet index rewrites embed the client Host and X-Forwarded-Proto into per-request JSON, so a shared cache could store one host's URLs, a forged header could inject content, HEAD skipped the rewrite, and immutable flat-container blobs were re-validated as mutable.
- set `no-store`, `Vary: Host, X-Forwarded-Proto` and the rewritten Content-Length on GET and HEAD
- share `v1.Scheme()` (first X-Forwarded-Proto value, `http`/`https` only) across nuget, cargo, virtual redirects and the terraform registry
- reject non host[:port] Host headers and JSON-escape the injected base URL
- classify `.snupkg`, `readme` and `icon` flat-container blobs as immutable
requires unkin/artifactapi#148 merged first
Reviewed-on: #149
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>
