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/coderdtest/authorize.go

@@ -272,6 +272,11 @@ func AGPLRoutes(a *AuthTester) (map[string]string, map[string]RouteCheck) {
 			AssertAction: rbac.ActionRead,
 			AssertObject: rbac.ResourceTemplate,
 		},
+
+		"GET:/api/v2/debug/coordinator": {
+			AssertAction: rbac.ActionRead,
+			AssertObject: rbac.ResourceDebugInfo,
+		},
 	}
 
 	// Routes like proxy routes support all HTTP methods. A helper func to expand