[C++] ExtensionType doesn't define default bit_width() and byte_width()

### Describe the enhancement requested

`ExtensionType` should implement `bit_width` and `byte_width` by delegating the call to the storage type.

```cpp
 int32_t byte_width() const override { return storage_type_->byte_width(); }
 int bit_width() const override { return storage_type_->bit_width(); }
```



### Component(s)

C++