Add ExtensionArray._scalar_from_string
the DatetimeArray PR is adding a `_scalar_from_string` for the datelike arrays. We could consider something similar for the EA interface.


```python
In [6]: pd.core.arrays.DatetimeArrayMixin(np.array([], dtype='i8'))._scalar_from_string("2000")
Out[6]: Timestamp('2000-01-01 00:00:00')

```