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)

docs/manifest.json

@@ -364,6 +364,10 @@
           "title": "Builds",
           "path": "./api/builds.md"
         },
+        {
+          "title": "Debug",
+          "path": "./api/debug.md"
+        },
         {
           "title": "Enterprise",
           "path": "./api/enterprise.md"