High memory usage when handling large responses (PDF generation use case)

### High memory usage when handling large responses (PDF generation use case)

**Description**  
When generating PDFs (large payload), SDK loads entire response into memory.

**Code snippet**

    result, err := apiClient.Request(ctx, "POST", baseURL, "/pdf", &buf, nil)

    fmt.Println(len(result))

**Issue**  
No streaming support (`io.Reader` response expected).