Consider adding isnative property to StringDtype?
#### Code Sample

Assuming Jupyter notebook environment

```bash
!pip install rpy2
```

```python
%load_ext rpy2.ipython
from pandas import *
characters= = Series(['a', 'b'], dtype=StringDtype())
%R -i characters
```

Leads to an error in rpy2:

```
AttributeError: 'StringDtype' object has no attribute 'isnative'`
```

which I described in https://github.com/rpy2/rpy2/issues/669. However, as this concerns a new experimental feature in pandas I wanted to bring a difference in public interface between different Dtypes to your attention.

#### Problem description

The new, experimental [`StringDtype`](https://github.com/pandas-dev/pandas/blob/845c50c9e2ce611c773422ae9db7097fc3e5fba5/pandas/core/arrays/string_.py#L27) inherits from [`ExtensionDtype`](https://github.com/pandas-dev/pandas/blob/dd03c196d684d6ed594ee58451b0a36d7935a1ea/pandas/core/dtypes/base.py#L17) rather than from [`PandasExtensionDtype`](https://github.com/pandas-dev/pandas/blob/e84c3f9abb32a10b6561489589a06a1a3c7623e0/pandas/core/dtypes/dtypes.py#L111). This means that it does not have `isnative` property.

#### Questions
- is it intended to have some Dtypes with `isnative` attribute and others without?
- in your opinion, should the rpy2 issue be worked around downstream or should the API be amended in pandas (I expect that other packages may also expect dtypes to have certain attributes from `PandasExtensionDtype`)?

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.1.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-26-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 1.0.3
numpy            : 1.18.2
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.2.3
setuptools       : 41.2.0
Cython           : None
pytest           : 5.4.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.13.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.2.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.4.1
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
