Allow per-route request handler timeouts and extending the timeout from the handler

[vdusek]

Python already separates the navigation and request handler timeouts, but request_handler_timeout applies uniformly to every request: Router.handler() (src/crawlee/router.py) takes no per-route override, and SharedTimeout (src/crawlee/_utils/time.py) offers no way to extend an already-running budget. Add both, so a single route can get a larger timeout and a handler can buy more time once it knows how much work it faces.

JS implementation: apify/crawlee#3860

✍️ Drafted by Claude Code