デフォルトの環境変数を超えて Go でプロキシ設定をプログラムで構成するにはどうすればよいですか?

<code class="go">import (
  "net/http"
  "net/http/httputil"
)

func main() {
  // Retrieve the proxy configuration from environment variables.
  proxyURL := httputil.ProxyFromEnvironment(nil)

  // Create a custom transport with the proxy configuration.
  transport := &http.Transport{
    Proxy: proxyURL,
  }

  // Initialize an HTTP client using the custom transport.
  client := &http.Client{
    Transport: transport,
  }

  // Perform an HTTP request using the proxied client.
  resp, err := client.Get("https://example.com")
  if err != nil {
    // Handle error
  }
  
  // Read the response body.
  bodyBytes, err := ioutil.ReadAll(resp.Body)
  if err != nil {
    // Handle error
  }
  
  bodyString := string(bodyBytes)
  fmt.Println("Response body:", bodyString)
}</code>