Add an `extend_context` option for adding members to the crawling context

[vdusek]

ContextPipeline is internal, so there is no supported way to put custom members on the crawling context - users have to subclass the crawler. JS v4 exposes an extendContext option that runs before navigation, making its members visible to the navigation hooks and the request handler alike, and additionally lets navigation hooks override context members.

JS implementation: apify/crawlee#3119, apify/crawlee#3672

✍️ Drafted by Claude Code

[janbuchar]

I'm not sure if this is possible with Python's type system.

[vdusek]

Yeah, I opened issues for all the gaps in Crawlee JS v4 and Crawlee Python. We can triage them once v2 is here, and see whether and how we can resolve them.