feat(coderd/healthcheck): add health check for proxy (#10846)

Adds a health check for workspace proxies:
- Healthy iff all proxies are healthy and the same version,
- Warning if some proxies are unhealthy,
- Error if all proxies are unhealthy, or do not all have the same version.

docs/api/debug.md

@@ -253,6 +253,39 @@ curl -X GET http://coder-server:8080/api/v2/debug/health \
     "healthy": true,
     "severity": "ok",
     "warnings": ["string"]
+  },
+  "workspace_proxy": {
+    "error": "string",
+    "healthy": true,
+    "severity": "ok",
+    "warnings": ["string"],
+    "workspace_proxies": {
+      "regions": [
+        {
+          "created_at": "2019-08-24T14:15:22Z",
+          "deleted": true,
+          "derp_enabled": true,
+          "derp_only": true,
+          "display_name": "string",
+          "healthy": true,
+          "icon_url": "string",
+          "id": "497f6eca-6276-4993-bfeb-53cbbbba6f08",
+          "name": "string",
+          "path_app_url": "string",
+          "status": {
+            "checked_at": "2019-08-24T14:15:22Z",
+            "report": {
+              "errors": ["string"],
+              "warnings": ["string"]
+            },
+            "status": "ok"
+          },
+          "updated_at": "2019-08-24T14:15:22Z",
+          "version": "string",
+          "wildcard_hostname": "string"
+        }
+      ]
+    }
   }
 }
 ```