feat: support to set the file mod

[LinuxSuRen]

Usage: hd get https://xxx --mod 0755

[codecov]

Codecov Report

Merging #347 (a869f98) into master (6c591a1) will increase coverage by 1.98%.
The diff coverage is 74.25%.

@@            Coverage Diff             @@
##           master     #347      +/-   ##
==========================================
+ Coverage   58.13%   60.12%   +1.98%     
==========================================
  Files          63       63              
  Lines        3258     3330      +72     
==========================================
+ Hits         1894     2002     +108     
+ Misses       1268     1226      -42     
- Partials       96      102       +6     

Flag	Coverage Δ
unittests	60.12% <74.25%> (+1.98%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cmd/install.go	34.74% <33.33%> (-0.18%)	⬇️
pkg/installer/check.go	35.59% <40.00%> (+0.05%)	⬆️
cmd/fetch.go	72.22% <50.00%> (+1.06%)	⬆️
cmd/get.go	69.32% <61.53%> (-0.56%)	⬇️
pkg/os/generic_installer.go	61.88% <72.22%> (+1.41%)	⬆️
cmd/root.go	81.39% <100.00%> (+0.44%)	⬆️
cmd/setup.go	78.72% <100.00%> (+0.94%)	⬆️
pkg/exec/command.go	86.66% <100.00%> (+78.55%)	⬆️
pkg/net/http.go	80.76% <100.00%> (+0.93%)	⬆️
... and 1 more

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