add support:not all tracker connection use proxy server

[Tearon2020]

i have lots of pt websites,because of isp restrict,some trackers only could connect successful by using proxy server.
befor, i use two qbittorrent clients for my pt. one use proxy, the other connect without proxy.
so i want to need custom proxy rule.
if the libtorrent support, i would only use one qbittorrent.

[arvidn]

I'm a bit uneasy about this. It's a pretty bug feature for a (presumably) rare use case.
There's a global setting to proxy_tracker_connections, maybe it would be more appropriate to have a per-tracker setting to override this. I don't think there's any precedence for that though.

[Tearon2020]

proxy_tracker_connections

i known this option,but it doesn't useful for me.
example: i have 6 pt websites， A/B/C/D/E can connect tracker direct， but F can not， i only use another client connect it with proxy setting.
although ,i could use only one client for all pt website with proxy, but some pt website will judge it seedbox, because the proxy ip is vps.
so, i priority use direct first.