Add Spinner, a Minecraft hostname-routing reverse proxy

Routes Java Edition connections by the hostname in the handshake, with
wildcard domains, backend failover, PROXY protocol v2, cached server-list
status from health probes, hot-reloaded config, and an admin page and
metrics endpoint. Includes CI, tagged releases and a container image.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

web/web.go

@@ -0,0 +1,7 @@
+// Package web embeds the admin UI.
+package web
+
+import _ "embed"
+
+//go:embed index.html
+var Index []byte