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>

internal/proxy/engine.go

@@ -353,7 +353,7 @@ func (e *Engine) headUpstreamOnce(ctx context.Context, remote models.Remote, pat
 				req.Header.Set(k, v)
 			}
 		}
-		return http.DefaultClient.Do(req)
+		return clientForRemote(remote).Do(req)
 	}
 
 	resp, err := doHead(nil)

internal/proxy/engine_test.go

@@ -219,6 +219,20 @@ func TestHead(t *testing.T) {
 	}
 }
 
+func TestHeadUpstreamUsesRemoteTimeout(t *testing.T) {
+	slow := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		time.Sleep(3 * time.Second)
+	}))
+	defer slow.Close()
+	r := models.Remote{Name: "slow-head", PackageType: models.PackageGeneric, BaseURL: slow.URL, UpstreamResponseHeaderTimeout: 1}
+
+	start := time.Now()
+	_, err := (&Engine{}).headUpstreamOnce(context.Background(), r, "blob.bin", prov(t, models.PackageGeneric))
+	if err == nil || time.Since(start) > 2*time.Second {
+		t.Fatalf("HEAD ignored the remote's response header timeout: err=%v after %s", err, time.Since(start))
+	}
+}
+
 func TestStaleOnError(t *testing.T) {
 	requireStack(t)
 	ctx := context.Background()