Clean up ClientConfig HTTP options

There are a proliferation of HTTP related options in ClientConfig. I believe it might be possible to converge on just exposing *http.Client for each use case. *http.Client handles having custom proxies, TLS settings, redirect handling, timeouts etc.

The use cases for HTTP in anacrolix/torrent are:

- Trackers
- WebRTC
- Sources (fetching torrent files)
- Webseeds
- ??

One strong holdout against this I think is the websocket library in use for webtorrent trackers. It only takes a proxy option. However the nyooyr.io/websocket module takes *http.Client, so this could be upgraded.

Another place is resolving IPs for trackers. This is currently done manually to check against blocklists before issuing HTTP requests. I think this is still doable, but some APIs might change.