BUG: Named aggregations do not work together with unnamed ones
### 

- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.

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


### Reproducible Example

```python
import pandas as pd
import numpy as np

groupbyObj = pd.DataFrame({"A": [1, 2, 3], "B": [10, 5, 1], "C": ["x", "x", "y"]}).groupby("C")
methods = {
    "A": np.mean,
    "NewName": pd.NamedAgg(column='B', aggfunc='max')
}

# now regardless if you pass it as a dict...
groupbyObj.agg(methods)

# ... or as keyword arguments, it fails
groupbyObj.agg(**methods)
```


### Issue Description

There seems to be no way to include both the unnamed and named aggregations as (an) argument(s) in the same groupby `agg`. It might be convenient and useful to name just a few specific aggregations rather than all or none of them. Currently, pandas behaviour leads to either a `KeyError: "Column(s) ['NewName'] do not exist"` or a `TypeError: Must provide 'func' or tuples of '(column, aggfunc).`

### Expected Behavior

Ideally, running
```
groupbyObj.agg(**methods)
```
would work instead of leading to an error.

### Installed Versions

<details>

INSTALLED VERSIONS
------------------
commit           : f00ed8f47020034e752baf0250483053340971b0
python           : 3.9.6.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.19042
machine          : AMD64
processor        : Intel64 Family 6 Model 140 Stepping 1, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : Polish_Poland.1250

pandas           : 1.3.0
numpy            : 1.21.0
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.2.1
setuptools       : 56.0.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 3.0.1
IPython          : 7.25.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.4.2
numexpr          : None
odfpy            : None
openpyxl         : 3.0.7
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.7.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
