Add TransferSubGroup to transfer a group to a new parent group

[gro-gg]

this fixes #1466

[gro-gg]

@svanharmelen

I have moved a group back and forth with this test:

package gitlab

import (
	"fmt"
	"os"
	"testing"
)

func TestName(t *testing.T) {
	parent1 := 52852274
	//parent2 := 52852276
	child := 52852277
	token := os.Getenv("GITLAB_TOKEN_RW")
	client, err := NewClient(token)
	if err != nil {
		t.Fatal(err)
	}

	opt := &TransferSubGroupOptions{
		ParentID: Int(parent1),
	}

	group, response, err := client.Groups.TransferSubGroup(child, opt)
	if err != nil {
		t.Fatal(err)
	}
	fmt.Println(group)
	fmt.Println(response)
}

[svanharmelen]

So close 😉

[svanharmelen]

Thanks @gro-gg 👍🏻

[gro-gg]

🙇 Thanks for reviewing and merging!

@svanharmelen When do you plan to release a new version? 😊
I need this new feature in: gro-gg/terraform-provider-gitlab@5387a58

[svanharmelen]

Just released v0.65.0