Implement `reqwest` based HTTP client in linkchecker

[AMDmi3]

Then it can be ran parallel to python client, results compared and logged.

[AMDmi3]

Rust client has been running in parallel to python client for some weeks on a subset (chosen in a way to most likely hit a discrepancy, e.g. we only run native client on 0.1% of 200 responses, but on 100% of error responses) of checks, and it looks great so far.

It reports different status in <0.1% cases (and that's a tiny fraction of all links), most of which are false positives (flaky connection refused, 5xx etc.), and some are caused by different low level libraries used, which we cannot affect (ConnectionAborted ⇄ ServerDisconnected ⇄ ConnectionResetByPeer, or different SSL error handling).

There are a few cases where rust client shows more correct (or at least less problematic) behavior than aiohttp:

• https://pyropus.ca. (host which ends with a period, this is a valid hostname)
• https://git.lighttpd.net/lighttpd/fcgi-cgi.git/snapshot/fcgi-cgi-0.2.2.tar.gz, http://www.fefe.de/dietlibc (aiohttp errors)

And there is one potential problem with Cloudflare (for instance, https://akson.ai/). Urls like that work fine in browser and ssllabs does not report any problems, however rustls reports UnknownIssuer. It may be that the cause is Extra download intermediate certificate reported by sslabs. I plan to check whether all or just a fraction of cloudflare urls are affected.

Additionally, there's discrepancy with hickory-resolver (versus aiodns used in prev-gen checker), which refuses hostnames with underscores such as human-friendly_pedantic-timedelta.readthedocs.io. Is far as I know, such hostnames are invalid, but most other clients handle them fine. I've only seen readthedocs.io cases so far, and readthedocs itself reply with 400 for these domains, so this may not really be a problem.

[AMDmi3]

(cloudflare problem seems to be solved by switching to native root cert store)

[AMDmi3]

List of individual issues (these do not block the switch to native client, and may indicate problem either in it or a server)

• https://www.vectorcamp.gr/project/vectorscan/ - probably due to line folding (multiline headers)
• ConnectionReset's all caused by Microsoft IIS
  • https://www.stardock.com/products/fences
  • https://ultraviewer.net/en/contact.html
  • https://submain.com/products/ghostdoc.aspx
  • https://windows.php.net/downloads/releases/php-8.4.4-Win32-vs17-x64.zip
  • https://www.gdiobjects.com/Files/PDWSInstall.exe
  • https://www.lfd.uci.edu/~gohlke/
• http://www.cs.nott.ac.uk/~bmv/Dung/ - dropping connection after lowercase headers
• a bunch of servers not supporting tls1.3
• http://flatassembler.net/fasm-1.73.04.tar.gz - looks like custom server software, may be incompatible