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>

spinner.example.json

@@ -0,0 +1,24 @@
+{
+  "bind": ":25565",
+  "admin_bind": "127.0.0.1:8080",
+  "handshake_timeout_secs": 5,
+  "connect_timeout_secs": 3,
+  "probe_interval_secs": 10,
+  "max_conns_per_ip": 20,
+  "unknown_motd": "§cUnknown server\n§7Check the address you connected to",
+  "servers": [
+    {
+      "name": "survival",
+      "domains": ["survival.example.com", "*.survival.example.com"],
+      "addresses": ["127.0.0.1:25566"],
+      "send_proxy_protocol": false,
+      "offline_motd": "§eSurvival is sleeping\n§7Try again in a moment",
+      "max_connections": 100
+    },
+    {
+      "name": "creative",
+      "domains": ["creative.example.com", "localhost"],
+      "addresses": ["127.0.0.1:25567", "10.0.0.5:25565"]
+    }
+  ]
+}