Add ansible virtual repositories (#158)

Clients need one Galaxy server to install both internal collections and galaxy.ansible.com ones; ansible has no virtual type.

- merge member collection version lists in member order, the earlier member winning a duplicate version, cached with the shared virtual merge cache
- paginate limit/offset over the merged list, following remote member pages
- serve version detail from the owning member with href and download_url via the virtual; downloads 302 to the member through the member-redirect validation
- fail closed with 502 on a member error or a member versions list shorter than its count
- add docs/ansible/virtual.md and an e2e-docker case driving real ansible-galaxy

requires unkin/artifactapi#157

Reviewed-on: #158
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

internal/api/v1/proxy.go

@@ -211,6 +211,10 @@ func (h *ProxyHandler) handleVirtual(w http.ResponseWriter, r *http.Request) {
 		h.serveVirtualGems(w, r, *virt, path, proxyBaseURL)
 		return
 	}
+	if virt.PackageType == models.PackageAnsible {
+		h.serveVirtualAnsible(w, r, *virt, path, proxyBaseURL)
+		return
+	}
 
 	loc, ok, err := h.virtualEngine.MemberRedirect(r.Context(), *virt, path, r.URL.RawQuery, proxyBaseURL)
 	if err != nil {