feat(binding): add support for encoding.UnmarshalText in uri/query binding

[takanuva15]

With this PR, users can now specify parser=encoding.TextUnmarshaler in their form/uri tag to enable automatic binding using the encoding.TextUnmarshaler interface from the Golang standard library. Since this new parser tag is 100% opt-in only, this is fully backwards-compatible with previous versions of gin.

This PR resolves a number of tickets regarding this functionality:

• Add support for UnmarshalText from Golang encoding.TextUnmarshaler for URI and Query param binding #4177
• feat(binding): Support custom BindUnmarshaler for binding. #3933 (comment)
• TextUnmarshaler support for binding #3045

closes #4177

---

Sample code to run:

package main

import (
  "encoding"
  "strings"

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

type Birthday string

func (b *Birthday) UnmarshalText(text []byte) error {
  *b = Birthday(strings.Replace(string(text), "-", "/", -1))
  return nil
}

var _ encoding.TextUnmarshaler = (*Birthday)(nil) //assert Birthday implements encoding.TextUnmarshaler

func main() {
  route := gin.Default()
  var request struct {
    Birthday         Birthday   `form:"birthday,parser=encoding.TextUnmarshaler"`
    Birthdays        []Birthday `form:"birthdays,parser=encoding.TextUnmarshaler" collection_format:"csv"`
    BirthdaysDefault []Birthday `form:"birthdaysDef,default=2020-09-01;2020-09-02,parser=encoding.TextUnmarshaler" collection_format:"csv"`
  }
  route.GET("/test", func(ctx *gin.Context) {
    _ = ctx.BindQuery(&request)
    ctx.JSON(200, request)
  })
  _ = route.Run(":8088")
}

Test it with:

curl 'localhost:8088/test?birthday=2000-01-01&birthdays=2000-01-01,2000-01-02'

Result

{"Birthday":"2000/01/01","Birthdays":["2000/01/01","2000/01/02"],"BirthdaysDefault":["2020/09/01","2020/09/02"]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.00%. Comparing base (3dc1cd6) to head (842bd59).
Report is 96 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4203      +/-   ##
==========================================
- Coverage   99.21%   99.00%   -0.21%     
==========================================
  Files          42       44       +2     
  Lines        3182     3423     +241     
==========================================
+ Hits         3157     3389     +232     
- Misses         17       24       +7     
- Partials        8       10       +2     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags "sonic avx"	98.99% <100.00%> (?)
-tags go_json	98.99% <100.00%> (?)
-tags nomsgpack	98.99% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.23	99.00% <100.00%> (?)
go-1.24	99.00% <100.00%> (?)
macos-latest	99.00% <100.00%> (-0.21%)	⬇️
ubuntu-latest	99.00% <100.00%> (-0.21%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[takanuva15]

@appleboy ready for review. I verified locally that all my new changes are covered with the new tests I added.