Building a multipart/form-data body outside Client::Post/Put/Patch
(e.g. to feed a custom content provider or compose with other body
sources) previously required calling several detail:: functions
directly, which is not a stable interface.
Introduce MultipartFormDataWriter, a small public class that owns the
boundary and delegates to the existing detail:: serializers: whole-body
serialization with known content length, and per-part framing
(item_begin/item_end/finish) for streaming. Also expose
is_valid_multipart_boundary() so callers using an explicit boundary can
validate it without exceptions.