Add PythonProtocol class that mirrors H1CProtocol callback interface:
- Callbacks: on_message_begin, on_url, on_header, on_headers_complete,
on_body, on_message_complete
- Properties: method, path, http_version, headers, content_length,
is_chunked, should_keep_alive
- Methods: feed(data), reset()
- Supports Content-Length and chunked transfer encoding
Add CallbackRequest adapter for building requests from parser state.
Works with both H1CProtocol (C extension) and PythonProtocol.
Add unit tests for PythonProtocol and CallbackRequest.
