Support slice and array of BindUnmarshaler

[jmfederico]

Allows form binding of []BindUnmarshaler and [x]BindUnmarshaler

Example usage:

package main

import (
	"strconv"

	"github.com/gin-gonic/gin"
)

type customUnmarshalParamHex int

func (f *customUnmarshalParamHex) UnmarshalParam(param string) error {
	v, err := strconv.ParseInt(param, 16, 64)
	if err != nil {
		return err
	}
	*f = customUnmarshalParamHex(v)
	return nil
}

func main() {
	route := gin.Default()
	var request struct {
		HexSlice []customUnmarshalParamHex `form:"[]hex_slice"`
	}
	route.GET("/test", func(ctx *gin.Context) {
		_ = ctx.BindQuery(&request)
		ctx.JSON(200, request.HexSlice)
	})
	route.Run(":8088")
}

Test with:

curl 'localhost:8088/test?[]hex_slice=f5&[]hex_slice=f6'

Result:

"[245,246]"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.06%. Comparing base (3dc1cd6) to head (de9a9eb).
Report is 65 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3988      +/-   ##
==========================================
- Coverage   99.21%   99.06%   -0.15%     
==========================================
  Files          42       44       +2     
  Lines        3182     2785     -397     
==========================================
- Hits         3157     2759     -398     
+ Misses         17       15       -2     
- Partials        8       11       +3     

Flag	Coverage Δ
	?
-tags "sonic avx"	99.05% <100.00%> (?)
-tags go_json	99.05% <100.00%> (?)
-tags nomsgpack	99.05% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	99.06% <100.00%> (-0.15%)	⬇️
go-1.22	99.06% <100.00%> (?)
macos-latest	99.06% <100.00%> (-0.15%)	⬇️
ubuntu-latest	99.06% <100.00%> (-0.15%)	⬇️

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[appleboy]

testing failed