Feature Request: Support ADC (Application Default Credentials) for Google Cloud Storage

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Problem

Currently, GCS_CREDENTIALS (base64-encoded service account JSON) is required to use GCS storage. If not provided, initialization fails.

https://github.com/langgenius/dify-cloud-kit/blob/main/oss/gcsblob/gcs.go#L30-L35

credentials, err := base64.StdEncoding.DecodeString(credentialsB64)
if err != nil {
    return nil, errors.New("credentials must be a base64 encoded string")
}
client, err := storage.NewClient(ctx, option.WithCredentialsJSON(credentials))

Use Case

When running on GKE with Workload Identity or GCE with attached service accounts, ADC (Application Default Credentials) is the recommended authentication method. It eliminates the need to manage and rotate service account keys manually.

Proposed Solution

var client *storage.Client
if credentialsB64 == "" {
    // Use ADC when credentials not provided
    client, err = storage.NewClient(ctx)
    if err != nil {
        return nil, errors.New("failed to initialize GCS client: ADC unavailable and no credentials provided")
    }
} else {
    credentials, err := base64.StdEncoding.DecodeString(credentialsB64)
    if err != nil {
        return nil, errors.New("credentials must be a base64 encoded string")
    }
    client, err = storage.NewClient(ctx, option.WithCredentialsJSON(credentials))
}

This allows:

• Existing behavior when GCS_CREDENTIALS is set
• ADC fallback when GCS_CREDENTIALS is empty