feat: add SampleBy and SamplesBy

[bramvandewalle]

Why?

See #512

What?

Added a SampleBy and SamplesBy that accepts an intFunc that is the generator of the indexes. This way, a client of the lo library would be able to provide their own seeded random generator.

Example?

package main

import (
	"fmt"
	"math/rand"

	"github.com/samber/lo"
)

func Example() {
	r := rand.New(rand.NewSource(42))
	collection := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
	result := lo.SampleBy(collection, r.Intn)
	results := lo.SamplesBy(collection, len(collection), r.Intn)
	fmt.Printf("Sample: %v\n", result)
	fmt.Printf("Samples: %v\n", results)
}

func main() {
	Example()
}

[ccoVeille]

Why this method doesn't call SamplesBy with count=1 and return the first entry?

[bramvandewalle]

Also the original implementation of the Sample doesn't use Samples. I didn't want to change any implementation of existing functions. Also... not sure if there's that much of a benefit in doing so?

[bramvandewalle]

But if you want me to update it... by all means and I change it!

[ccoVeille]

Let's wait for owner's feedback

cc @samber

[ccoVeille]

I posted this, but feel free to merge.

[bramvandewalle]

@ccoVeille I don't have the rights to merge this. Not sure if @samber first needs to approve it?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.15%. Comparing base (dc8a175) to head (4e7903d).
Report is 46 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #516      +/-   ##
==========================================
- Coverage   94.33%   94.15%   -0.18%     
==========================================
  Files          17       18       +1     
  Lines        2790     3065     +275     
==========================================
+ Hits         2632     2886     +254     
- Misses        154      168      +14     
- Partials        4       11       +7     

Flag	Coverage Δ
unittests	94.15% <100.00%> (-0.18%)	⬇️

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