feat: add debug server for tailnet coordinators (#5861)

Implements a Tailscale-like debug server for our in-memory coordinator. This should provide some visibility into why connections could be failing.
Resolves: https://github.com/coder/coder/issues/5845

![image](https://user-images.githubusercontent.com/6332295/214680832-2724d633-2d54-44d6-a7ce-5841e5824ee5.png)

coderd/coderd.go

@@ -613,6 +613,25 @@ func New(options *Options) *API {
 				r.Get("/", api.workspaceApplicationAuth)
 			})
 		})
+
+		r.Route("/debug", func(r chi.Router) {
+			r.Use(
+				apiKeyMiddleware,
+				// Ensure only owners can access debug endpoints.
+				func(next http.Handler) http.Handler {
+					return http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
+						if !api.Authorize(r, rbac.ActionRead, rbac.ResourceDebugInfo) {
+							httpapi.ResourceNotFound(rw)
+							return
+						}
+
+						next.ServeHTTP(rw, r)
+					})
+				},
+			)
+
+			r.Get("/coordinator", api.debugCoordinator)
+		})
 	})
 
 	if options.SwaggerEndpoint {