DataFrameGroupBy.aggregate engine='numba' doesn't pass index
- [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.

---

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

```python
df = pd.DataFrame({'group': ['A', 'A', 'B'], 'v': [4, 5, 6]}, index=[-1, -2, -3])

def f(values, index):
    return np.mean(index)
df.groupby('group').aggregate(f, engine='numba')

# Result is:
#          v
# group     
# A      0.5
# B      2.0
```

#### Problem description

The behaviour when engine='numba' is inconsistent with engine='cython':
```python
def f_cython(x):
    return np.mean(x.index)
df.groupby('group').aggregate(f_cython, engine='cython')

# Result is:
#          v
# group     
# A      -1.5
# B      -3.0
```
Furthermore engine='numba' would be more useful if it calls func with the DataFrame index.

#### Expected Output
```
         v
group     
A      -1.5
B      -3.0
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : c7f7443c1bad8262358114d5e88cd9c8a308e8aa
python           : 3.7.3.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.0-80-generic
Version          : #90-Ubuntu SMP Fri Jul 9 22:49:44 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 1.3.1
numpy            : 1.20.3
pytz             : 2021.1
dateutil         : 2.8.2
pip              : 19.2.3
setuptools       : 52.0.0.post20210125
Cython           : 0.29.24
pytest           : 6.2.4
hypothesis       : 6.14.1
sphinx           : 4.0.2
blosc            : None
feather          : None
xlsxwriter       : 3.0.1
lxml.etree       : 4.6.3
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 3.0.1
IPython          : 7.26.0
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : 1.3.2
fsspec           : 2021.07.0
fastparquet      : None
gcsfs            : None
matplotlib       : 3.1.1
numexpr          : 2.7.3
odfpy            : None
openpyxl         : 3.0.7
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.6.2
sqlalchemy       : 1.4.22
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : 2.0.1
xlwt             : 1.3.0
numba            : 0.53.1

</details>
