refactor:refactor RandomString function

[pigwantacat]

reference resources：How to generate a random string of a fixed length in Go?

func BenchmarkApproach1(b *testing.B) {
	for i := 0; i < b.N; i++ {
		_ = RandomString(1<<10, lo.AlphanumericCharset)
	}
}

func BenchmarkApproach2(b *testing.B) {
	for i := 0; i < b.N; i++ {
		_ = lo.RandomString(1<<10, lo.AlphanumericCharset)
	}
}

$ go test -benchmem -benchtime=5s -bench .
goos: windows
goarch: amd64
pkg: mytttttt
cpu: Intel(R) Core(TM) i5-10400 CPU @ 2.90GHz
BenchmarkApproach1-12            1000000              5423 ns/op            1024 B/op          1 allocs/op
BenchmarkApproach2-12             292579             20485 ns/op            5248 B/op          2 allocs/op
PASS
ok      mytttttt        12.160s

[samber]

that's cool!

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 84.61538% with 6 lines in your changes missing coverage. Please review.

Project coverage is 94.33%. Comparing base (bc0037c) to head (e8bb837).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
string.go	84.61%	4 Missing and 2 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #524      +/-   ##
==========================================
- Coverage   94.48%   94.33%   -0.15%     
==========================================
  Files          17       17              
  Lines        2757     2790      +33     
==========================================
+ Hits         2605     2632      +27     
- Misses        150      154       +4     
- Partials        2        4       +2     

Flag	Coverage Δ
unittests	94.33% <84.61%> (-0.15%)	⬇️

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