BUG: pandas.Series.round not yet implemented for FloatingArray
- [X ] I have checked that this issue has not already been reported.

- [ X] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
import pandas as pd
df = pd.DataFrame([{'a':1.2},{'a':2.5}])
df = df.convert_dtypes()
df['a'] = df['a'].round()
```

#### Problem description

Getting an error when applying `round` to a Series with new FloatingArray. 

```python
import pandas as pd
df = pd.DataFrame([{'a':1.2},{'a':2.5}])
df = df.convert_dtypes()
df['a'] = df['a'].round()
```

The error message is:
```
File ".../lib/python3.8/site-packages/pandas/core/series.py", line 2183, in round
    result = self._values.round(decimals)
AttributeError: 'FloatingArray' object has no attribute 'round'
```

#### Expected Output

rounded floats

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

INSTALLED VERSIONS
------------------
commit           : 3e89b4c4b1580aa890023fc550774e63d499da25
python           : 3.8.5.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 20.2.0
Version          : Darwin Kernel Version 20.2.0: Wed Dec  2 20:39:59 PST 2020; root:xnu-7195.60.75~1/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.0
numpy            : 1.19.4
pytz             : 2020.4
dateutil         : 2.8.1
pip              : 20.3
setuptools       : 50.3.2.post20201201
Cython           : None
pytest           : 6.1.2
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.19.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : 0.8.4
fastparquet      : None
gcsfs            : 0.7.1
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : 1.3.20
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
