fix: Expose impersonate flag on HTTP crawlers.

[Mantisus]

Description

• Expose an impersonate flag on the HTTP crawlers (HttpCrawler, BeautifulSoupCrawler, ParselCrawler) to turn browser impersonation on or off in the default ImpitHttpClient. The flag applies only to the default client; if a custom http_client is passed, it is ignored.
• Add a guide on working with HTTP headers in web scraping (docs/guides/http_headers.mdx) with a runnable example.

Issues

• Closes: Expose a browser-impersonation toggle directly on HTTP crawlers #1923

Testing

• Added new tests verifying the impersonate flag for all HTTP crawlers.

[Pijukatel]

Hi, I think it would be better to do just a documentation change and keep the current implementation.

I wrote the reasons into the issue, as the current wording of the issue is asking for a code change.

#1923 (comment)