add new socks5_alert to trouble shoot SOCKS5 proxies

[arvidn]

No description provided.

[codecov-io]

Codecov Report

Merging #4202 into RC_1_2 will decrease coverage by 0.12%.
The diff coverage is 39.39%.

@@            Coverage Diff             @@
##           RC_1_2    #4202      +/-   ##
==========================================
- Coverage    74.7%   74.57%   -0.13%     
==========================================
  Files         439      439              
  Lines       60537    60582      +45     
  Branches     8950     8963      +13     
==========================================
- Hits        45222    45182      -40     
- Misses      10829    10905      +76     
- Partials     4486     4495       +9

Impacted Files	Coverage Δ
test/test_priority.cpp	85.94% <ø> (-0.05%)	⬇️
include/libtorrent/udp_socket.hpp	77.77% <ø> (ø)	⬆️
include/libtorrent/aux_/session_udp_sockets.hpp	100% <ø> (ø)	⬆️
test/test_alert_types.cpp	100% <100%> (ø)	⬆️
src/alert.cpp	56.76% <100%> (+0.33%)	⬆️
include/libtorrent/alert_types.hpp	59.34% <100%> (+0.45%)	⬆️
src/session_udp_sockets.cpp	80.95% <100%> (ø)	⬆️
src/session_impl.cpp	56.09% <100%> (+0.44%)	⬆️
src/udp_socket.cpp	52.16% <20%> (-5.72%)	⬇️
test/test.cpp	48.14% <0%> (-40.75%)	⬇️
... and 29 more

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[arvidn]

I should expose the new alert to the python binding as well

[sledgehammer999]

Don't forget to add the new alert in fwd.hpp too.
In any case, I have created a test build and a new issue on qbt bug tracker to track all relevant progress. If you like, follow user reports here: qbittorrent/qBittorrent#11735