ot::Spinel::Encoder::WriteDataWithLen() method

Encodes and writes a data blob (sequence of bytes) with its length prepended before the data. The length of the data (in bytes) is prepended (with the length encoded as a `uint16`). The size of the length field is not included in the length. This is similar to `SPINEL_DATATYPE_DATA_WLEN` type. 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`.