Add WithRetry option for configurable request retries

Features

• New ClientOption type for configuring client behavior.
• WithRetry(retries int) option to set maximum retry attempts for failed GraphQL requests.
• No retry on HTTP URL error.
• Added comprehensive tests:
  • Retry behavior on temporary errors
  • Max retries exceeded scenarios
  • No retry on permanent errors

Usage

// Create client with 3 retry attempts
client := graphql.NewClient(
    "/graphql",
    httpClient,
    graphql.WithRetry(3),
)

var q struct {
    User struct {
        Name string
    }
}

// Query will automatically retry up to 3 times on temporary failures
err := client.Query(context.Background(), &q, nil)