feat: Add url field to PaginatedResponse and populate it using route path

Resolves: https://github.com/meta-llama/llama-stack/issues/1847
Changes:
- responses.py: added optional `url` field to PaginatedResponse
- server.py: automatically populate the URL field with route path

docs/_static/llama-stack-spec.html

@@ -10711,6 +10711,10 @@
                     "has_more": {
                         "type": "boolean",
                         "description": "Whether there are more items available after this set"
+                    },
+                    "url": {
+                        "type": "string",
+                        "description": "The URL for accessing this list"
                     }
                 },
                 "additionalProperties": false,