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/debug.go

@@ -0,0 +1,14 @@
+package coderd
+
+import "net/http"
+
+// @Summary Debug Info Wireguard Coordinator
+// @ID debug-info-wireguard-coordinator
+// @Security CoderSessionToken
+// @Produce text/html
+// @Tags Debug
+// @Success 200
+// @Router /debug/coordinator [get]
+func (api *API) debugCoordinator(rw http.ResponseWriter, r *http.Request) {
+	(*api.TailnetCoordinator.Load()).ServeHTTPDebug(rw, r)
+}