Expose CURL handle to write function via Handler

[bdbai]

This PR adds a new provided method write_context to Handler. In addition to the data buffer, it passes a context reference to the Handler which allows the Handler to act on the raw handle inside the callback.

I need this capability to as part of the ongoing WebSockets support. When working with the writefunction model, users should be able to get metadata from curl_ws_meta or perform a blocking send using curl_ws_send from inside the write function callback. With the context object, we can park these two methods under there.

While this PR is not directly related to WebSockets, the access to the raw handle would be handy in certain cases.