Proxy problem

Hi, i try my code to running inside our internal server, after i add the proxy i got error **Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential**

i already double check and try the env is same and correct like my local and is running without proxy

```
package firebase

import (
"context"
"encoding/base64"
firebase "firebase.google.com/go/v4"
google "google.golang.org/api/option"
"net/http"
"net/url"

"github.com/bytedance/sonic"

"go.brimola.bri.co.id/libs/config"
)

type (
Client struct {
*firebase.App
}
)

func NewClient(config *config.Config) (*Client, error) {
decodedPrivateKey, err := base64.StdEncoding.DecodeString(config.Message.FCM.PrivateKey)
if err != nil {
return nil, err
}

config.Message.FCM.PrivateKey = string(decodedPrivateKey)
serviceAccount, err := sonic.Marshal(config.Message.FCM)
if err != nil {
return nil, err
}

proxyURL, err := url.Parse(config.Message.FCM.Proxy)
if err != nil {
return nil, err
}

httpClient := &http.Client{
Transport: &http.Transport{
Proxy: http.ProxyURL(proxyURL),
},
}

firebaseAdmin, err := firebase.NewApp(context.Background(), nil, google.WithHTTPClient(httpClient), google.WithCredentialsJSON(serviceAccount))
if err != nil {
return nil, err
}

return &Client{firebaseAdmin}, nil
}
```