ot::Spinel::Encoder::EndFrame() method

Finalizes/ends the current frame being written to the buffer. Before using this method `BeginFrame()` must be called to start and prepare a new 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 frame and return error status `OT_ERROR_NO_BUFS`. Ensures to close any open structure (previously opened using `OpenStruct()` but not closed using `CloseStruct()`).