text: Title: Implement prepare_request/build_request and the corresponding Session.send()
  Body: **Is your feature request related to a problem? Please describe.**
  
  Currently, the `requests` api of `curl_cffi` is missing `prepare_request` or httpx's `build_request` method. The method is useful to construct request object and ability to view or edit the request object before sending the request via `Session().send` or `Client().send()`.
  
  **Describe the solution you'd like**
  
  httpx's `build_request` or requests' `prepare_request` method implemented in the requests api of curl_cffi
  
  **Describe alternatives you've considered**
  
  Using `curl_cffi` as `requests` adapter, so expected requests features are available.
  
  **Context**
  
  An example of the usefulness of `build_request` method in action.
  
  ```python
  client = requests.Session()
  # build_request is httpx method, similar to prepare_request
  req = client.build_request(method: 'GET', url, headers, cookies, data)
  # show request headers
  req.headers
  # Update user-agent header
  req.headers['User-Agent'] = 'my_web_crawler/0.1'
  # show request payload
  req.content
  # Update request payload using json string
  data =  { 'hello': 'world'}
  req.content = bytes(json.dumps(data), 'utf-8')
  # Update request method
  req.method = 'POST'
  # send the built request after fixing the issue
  resp = client.send(req)
  ```