feat: add ability to set custom multipart boundary value

[PokeGuys]

Use case

Integrating with some web service that require a specific boundary format in multipart/form-data requests.

Some legacy service might requires a specific prefix in the boundary value, to ensure the compatibility, we need the ability to set a custom boundary value in the multipart request.

Example

package main

import (
	"crypto/rand"
	"fmt"
	"io"
	"log"

	"github.com/go-resty/resty/v2"
)

const BoundaryPrefix = "Some_MultiPart_Prefix_"

func randomBoundary() string {
	var buf [4]byte
	_, err := io.ReadFull(rand.Reader, buf[:])
	if err != nil {
		panic(err)
	}
	return fmt.Sprintf("%s_%X", BoundaryPrefix, buf[:])
}

func main() {
    // Initialize the client
    client := resty.New()

    // Perform the file upload
    _, err := client.R().
        SetMultipartFormData(map[string]string{"test": "1"}).
        SetBoundary(randomBoundary()) // Setting the custom boundary
        Post("http://example.com/upload")
    if err != nil {
        log.Fatalf("Failed to upload file: %v", err)
    }
}

[jeevatkm]

@PokeGuys - Can you please add one practical use case and its example in the PR description?

[PokeGuys]

@jeevatkm Hi. Updated.

[jeevatkm]

@PokeGuys Thanks for adding the use case detail to the PR description.

Can you refactor this method as below and it includes updated godoc and version info too?

// SetMultipartBoundary method sets the custom multipart boundary for the multipart request.
// Typically, the `mime/multipart` package generates a random multipart boundary, if not provided.
//
// Since v2.15.0
func (r *Request) SetMultipartBoundary(boundary string) *Request {
 	r.multipartBoundary = boundary
 	return r
 }

[PokeGuys]

Updated.

[jeevatkm]

Thanks, @PokeGuys, for the PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.68%. Comparing base (231e19c) to head (9a5bfa7).
Report is 1 commits behind head on v2.

Additional details and impacted files

@@            Coverage Diff             @@
##               v2     #820      +/-   ##
==========================================
+ Coverage   96.67%   96.68%   +0.01%     
==========================================
  Files          14       14              
  Lines        1742     1748       +6     
==========================================
+ Hits         1684     1690       +6     
  Misses         37       37              
  Partials       21       21              

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