Gin Can UnmarshalParam support custom slice of string

Feature

Support UnmarshalParam for custom string slice type in tag form.

Description

If we define a custom type of string slice and implement the Gin's interface of UnmarshalParam, then we can't get the expected result.

How to reproduce

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")
}

Expectations

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

Actual result

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

Environment

• go version: go1.22.0 darwin/arm64
• gin version (or commit ref): v1.10.0
• operating system: macOS 13.5