Add gems virtual repositories (#154)

Bundler needs one source for internal and public gems, but gems virtuals have no merger, so a local gems repo and rubygems.org cannot be served together.

- merge `info/<gem>` across members (member order wins a duplicate version-platform); pass a single-member gem through verbatim
- regenerate `versions` lines and md5s for gems several members list; union `names`
- 302 `gems/*.gem` and `quick/Marshal.4.8/*` to the member owning that version, via the shared member-path validation
- return 502 on a member error; cache merges in the shared virtual cache; route HEAD on virtuals
- add `docs/gems/virtual.md` and a stock `ruby:3.3` bundler e2e-docker case

Reviewed-on: #154
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

internal/server/server.go

@@ -167,6 +167,7 @@ func (s *Server) routes() chi.Router {
 
 	proxyHandler := v1.NewProxyHandler(s.engine, s.virtEngine, s.db, s.store, s.localHandler)
 	s.virtEngine.NPMLocal = proxyHandler.LocalNPMPackument
+	s.virtEngine.GemsLocal = proxyHandler.LocalGemsIndex
 	r.Mount("/api/v1", proxyHandler.Routes())
 	r.Mount("/v2", proxyHandler.DockerV2Routes())