ot::Spinel::Encoder::WriteVPacked() method

Encodes and writes a set of variables to the current input frame using a given spinel packing format string. Before using this method `BeginFrame()` must be called to start and prepare a new input frame. Otherwise, this method does nothing and returns error status `OT_ERROR_INVALID_STATE`. If no buffer space is available, this method will discard and clear the current input frame and return the error status `OT_ERROR_NO_BUFS`. Note that the encoded buffer should fit in `kPackFormatBufferSize` bytes.