feature: support control max copy size for slice/array field

[zhuliquan]

You can limit array/slice type of field to a maximum of maxCopySize elements by using option func WithMaxCopyListSize(maxCopySize).

Usage:

package main

import (
    fieldmask_utils "github.com/mennanov/fieldmask-utils"
)

func main() {
    type A struct {
        Field []int
    }
    const copySize int = 2

    src := &A{Field: []int{1, 2, 3}}
    dst := &A{}
    mask := fieldmask_utils.MaskFromString("Field1")
    // Limit Field to copy up to copySize elements
    // After copy, dst is A{Field: []int{1,2}}
    err := fieldmask_utils.StructToMap(mask, src, dst,
        fieldmask_utils.WithMaxCopyListSize(copySize))
    if err != nil {
        panic(err)
    }
}

[codecov[bot]]

Codecov Report

Merging #24 (1b754f4) into master (a076f18) will increase coverage by 2.28%. The diff coverage is 85.00%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
+ Coverage   88.85%   91.14%   +2.28%     
==========================================
  Files           2        2              
  Lines         341      350       +9     
==========================================
+ Hits          303      319      +16     
+ Misses         20       16       -4     
+ Partials       18       15       -3     

Impacted Files	Coverage Δ
copy.go	88.79% <85.00%> (+3.59%)	:arrow_up:

---

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[zhuliquan]

I rewrite options with visitor function CopyListSize according to your advice.

New Usage:

package main

import (
    fieldmask_utils "github.com/mennanov/fieldmask-utils"
)

func main() {
    type A struct {
        Field []int
    }
    const copySize int = 2

    src := &A{Field: []int{1, 2, 3}}
    dst := &A{}
    mask := fieldmask_utils.MaskFromString("Field1")
    // Limit Field to copy up to copySize elements
    // After copy, dst is A{Field: []int{1,2}}
    err := fieldmask_utils.StructToStruct(mask, src, dst,
        fieldmask_utils.WithCopyListSize(func(src, dst *reflect.Value) int {
            return copySize
        }))
    if err != nil {
        panic(err)
    }
}

[mennanov]

Thanks a lot for contributing!