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

llama_stack/apis/common/responses.py

@@ -23,7 +23,9 @@ class PaginatedResponse(BaseModel):
 
     :param data: The list of items for the current page
     :param has_more: Whether there are more items available after this set
+    :param url: The URL for accessing this list
     """
 
     data: list[dict[str, Any]]
     has_more: bool
+    url: str | None = None