Implement a BFS web crawler

A web crawler starts from a URL and explores reachable pages level by level. You are given: - `start_url: string` - An interface `fetch_links(url: string) -> List[string]`, which returns all hyperlinks found on that page Write a function that returns all pages reachable from `start_url` that belong to the same hostname as `start_url`. Requirements: - Use breadth-first traversal. - Visit each URL at most once. - Ignore links whose hostname differs from the hostname of `start_url`. - `fetch_links` may return duplicate links. - You may return the result in BFS visitation order or in any order, as long as all valid reachable pages are included exactly once. Possible follow-up discussion: - How do you avoid revisiting the same page? - What are the time and space complexities?