refactor: add default client instance for contrib/sdk/httpclient

[windvalley]

用户在使用sdk/httpclient的时候可能忽略或忘记定制http客户端, 此时应该为其提供一个默认的http客户端实例(gclient.New()), 否则程序将会异常退出.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.21%. Comparing base (e0e0043) to head (e567ba9).
Report is 361 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2814      +/-   ##
==========================================
- Coverage   79.22%   79.21%   -0.02%     
==========================================
  Files         633      630       -3     
  Lines       52242    51903     -339     
==========================================
- Hits        41391    41117     -274     
+ Misses       8808     8760      -48     
+ Partials     2043     2026      -17     

Flag	Coverage Δ
go-1.15-386	79.18% <ø> (-0.03%)	⬇️
go-1.15-amd64	?
go-1.16-386	?
go-1.16-amd64	79.19% <ø> (-0.03%)	⬇️
go-1.17-386	?
go-1.17-amd64	?
go-1.18-386	?
go-1.18-amd64	?
go-1.19-386	?
go-1.19-amd64	79.18% <ø> (-0.04%)	⬇️
go-1.20-386	79.19% <ø> (+<0.01%)	⬆️
go-1.20-amd64	?

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[gqcn]

@windvalley Thanks for your contribution!