Send shadowsocks handshake with payload if available

[nekohasekai]

I am not sure if it makes sense

[codecov-commenter]

Codecov Report

Merging #1292 (30dc49f) into master (c278db5) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #1292   +/-   ##
=======================================
  Coverage   44.64%   44.65%           
=======================================
  Files         485      485           
  Lines       29524    29526    +2     
=======================================
+ Hits        13182    13184    +2     
- Misses      14939    14940    +1     
+ Partials     1403     1402    -1     

Impacted Files	Coverage Δ
proxy/shadowsocks/client.go	1.00% <0.00%> (-0.03%)	⬇️
common/drain/drainer.go	65.51% <0.00%> (-6.90%)	⬇️
transport/pipe/impl.go	87.87% <0.00%> (-2.03%)	⬇️
transport/internet/kcp/connection.go	72.37% <0.00%> (+0.55%)	⬆️
app/router/command/command.go	57.77% <0.00%> (+4.44%)	⬆️
app/router/command/errors.generated.go	100.00% <0.00%> (+100.00%)	⬆️

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