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>

docs/common/caching.md

@@ -95,7 +95,7 @@ Force a moving path mutable, e.g. branch tarballs on `generic`:
 Each type's page lists its auto-detected mutable and immutable paths:
 [alpine](../alpine/remote.md), [alpine github](../alpine/remote-github.md),
 [cargo](../cargo/remote.md), [deb](../deb/remote.md), [deb github](../deb/remote-github.md),
-[docker](../docker/remote.md), [generic](../generic/remote.md), [goproxy](../goproxy/remote.md),
+[docker](../docker/remote.md), [generic](../generic/remote.md), [gems](../gems/remote.md), [goproxy](../goproxy/remote.md),
 [helm](../helm/remote.md), [npm](../npm/remote.md), [nuget](../nuget/remote.md),
 [puppet](../puppet/remote.md), [pypi](../pypi/remote.md), [rpm](../rpm/remote.md),
 [rpm github](../rpm/remote-github.md), [terraform](../terraform/remote.md).

docs/gems/remote.md

@@ -0,0 +1,7 @@
+# `gems` remote
+
+Shared classification, TTL and eviction rules: [caching](../common/caching.md).
+
+| Type | Mutable (auto-detected) | Immutable (auto-detected) |
+|---|---|---|
+| `gems` | compact index (`versions`, `info/*`, `names`), `*specs.4.8.gz` | `gems/*.gem`, `quick/Marshal.4.8/*.gemspec.rz` |