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/apidoc/docs.go

@@ -362,6 +362,28 @@ const docTemplate = `{
                 }
             }
         },
+        "/debug/coordinator": {
+            "get": {
+                "security": [
+                    {
+                        "CoderSessionToken": []
+                    }
+                ],
+                "produces": [
+                    "text/html"
+                ],
+                "tags": [
+                    "Debug"
+                ],
+                "summary": "Debug Info Wireguard Coordinator",
+                "operationId": "debug-info-wireguard-coordinator",
+                "responses": {
+                    "200": {
+                        "description": "OK"
+                    }
+                }
+            }
+        },
         "/entitlements": {
             "get": {
                 "security": [