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>

.env.example

@@ -0,0 +1,3 @@
+# Bearer token for the admin UI/API and /metrics (admin is disabled if empty)
+SPINNER_ADMIN_TOKEN=change-me
+# SPINNER_CONFIG=spinner.json