Missing support for multipart/form-data (file upload use case)

### Missing support for multipart/form-data (file upload use case)

**Description**  
Cannot upload files using SDK in a typical Go service.

**Code snippet**

    body := &bytes.Buffer{}
    writer := multipart.NewWriter(body)

    // add file...

    _, err := apiClient.Request(ctx, "POST", baseURL, "/upload", body, nil)

**Issue**  
No way to set `Content-Type: multipart/form-data`.