feat(backend): add share_key field

[Integralist]

I tested this locally using the following script and was seeing an API validation error. This validation error was fixed internally by Fastly and now when running the script I get the expected valid! true output.

package main

import (
	"fmt"
	"log"
	"os"

	"github.com/fastly/go-fastly/v8/fastly"
)

func main() {
	client, err := fastly.NewClient(os.Getenv("FASTLY_API_KEY"))
	if err != nil {
		log.Fatal(err)
	}

	serviceID := "<REDACTED>"

	_, err = client.UpdateBackend(&fastly.UpdateBackendInput{
		Name:           "example-backend",
		ShareKey:       fastly.String("<REDACTED>"),
		ServiceID:      serviceID,
		ServiceVersion: 1,
	})
	if err != nil {
		log.Fatal(err)
	}

	valid, reason, err := client.ValidateVersion(&fastly.ValidateVersionInput{
		ServiceID:      serviceID,
		ServiceVersion: 1,
	})
	if err != nil {
		log.Fatal(err)
	}
	if !valid {
		log.Fatalf("not valid version '%s': %s", reason, err)
	}

	fmt.Printf("valid! %#v\n", valid)
}