ot::Spinel::Encoder::OpenStruct() method

Opens a struct in the current input frame. After a successful call to this method, all the subsequent `Write()` methods add the field/value to the current open struct until the struct is closed using `CloseStruct()` method. Structures can be nested. Up to `kMaxNestedStructs` nested structs can be opened at the same time. 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 frame and return error status `OT_ERROR_NO_BUFS`.