feat(form): add custom string slice for form tag unmarshal (#3970)

[bruceNu1l]

Resolves #3970

package main

import (
	"fmt"
	"net/http"
	"strings"

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

type CustomPath []string

func (b *CustomPath) UnmarshalParam(param string) error {
	elems := strings.Split(param, "/")
	n := len(elems)
	if n < 2 {
		return fmt.Errorf("invalid path: %s", param)
	}

	*b = elems
	return nil
}

type PathRequest struct {
	Paths CustomPath `form:"path"`
}

func main() {
	g := gin.Default()
	g.GET("", func(c *gin.Context) {
		var request PathRequest
		if err := c.ShouldBind(&request); err != nil {
			c.String(http.StatusBadRequest, "request parse err: %v", err)
			return
		}

		c.String(200, "Hello %s", request.Paths)
	})
	g.Run(":9000")
}



Calling this server with a string parameters path will get the expected slices.

Example: 
$ curl 'http://127.0.0.1:9000?path=hello/world' 
CustomPath:  [hello world]

[bruceNu1l]

format the code for better review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.05%. Comparing base (3dc1cd6) to head (5e4591f).
Report is 59 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3971      +/-   ##
==========================================
- Coverage   99.21%   99.05%   -0.16%     
==========================================
  Files          42       44       +2     
  Lines        3182     2755     -427     
==========================================
- Hits         3157     2729     -428     
+ Misses         17       15       -2     
- Partials        8       11       +3     

Flag	Coverage Δ
	?
-tags "sonic avx"	99.04% <100.00%> (?)
-tags go_json	99.04% <100.00%> (?)
-tags nomsgpack	99.04% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	99.05% <100.00%> (-0.16%)	⬇️
go-1.22	99.05% <100.00%> (?)
macos-latest	99.05% <100.00%> (-0.16%)	⬇️
ubuntu-latest	99.05% <100.00%> (-0.16%)	⬇️

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[RedCrazyGhost]

please add test case

[bruceNu1l]

please add test case

Test cases have been added, please review them again. Thank you.

[Apipa169]

Is it possible to add this for arrays as well? A use case for me would be converting a UUID to a binary uuid, which is a [16]byte

[bruceNu1l]

Is it possible to add this for arrays as well? A use case for me would be converting a UUID to a binary uuid, which is a [16]byte

Yes. There will be some custom arrays for business purposes, such as Mongo's ObjectID, which is a [12]byte.