Add HEAD route for remotes and gems remote type (#148)

Remotes answer only GET, so HEAD probes get 405, and there is no RubyGems remote. Synthesized (github_*) and rewritten (npm, helm, pypi, puppet, terraform, nuget) mutable responses need the GET path for HEAD lengths to match.

- add a HEAD route for /api/v1/remote/{name}/*; rewritten and synthesized mutable HEADs go through the GET path
- mark rewriting providers with a ResponseRewriter capability; set Content-Length on proxied GETs
- use the remote's upstream client for upstream HEAD
- add a gems remote type: compact index and specs mutable, .gem and gemspec immutable
- add gems docs, UI snippet and e2e caching case; add HEAD-vs-GET e2e checks

Reviewed-on: #148
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

ui/src/packagetypes/defs/gems.ts

@@ -0,0 +1,17 @@
+import type { PackageTypeUI } from '../types';
+
+const def: PackageTypeUI = {
+  color: 'red',
+  snippets: ({ proxy }) => {
+    return [
+      {
+        title: 'Use this remote as the RubyGems source',
+        language: 'bash',
+        code: `gem sources --clear-all --add ${proxy}/
+bundle config mirror.https://rubygems.org ${proxy}`,
+      },
+    ];
+  },
+};
+
+export default def;