BUG: Lacking `milliseconds` field in `DateOffset`
- [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
# Your code here
>>> import pandas as pd
>>> pd.date_range(start='2000-01-01 08:00:00', end='2000-01-01 08:00:01', freq='1L')
DatetimeIndex([       '2000-01-01 08:00:00', '2000-01-01 08:00:00.001000',
               '2000-01-01 08:00:00.002000', '2000-01-01 08:00:00.003000',
               '2000-01-01 08:00:00.004000', '2000-01-01 08:00:00.005000',
               '2000-01-01 08:00:00.006000', '2000-01-01 08:00:00.007000',
               '2000-01-01 08:00:00.008000', '2000-01-01 08:00:00.009000',
               ...
               '2000-01-01 08:00:00.991000', '2000-01-01 08:00:00.992000',
               '2000-01-01 08:00:00.993000', '2000-01-01 08:00:00.994000',
               '2000-01-01 08:00:00.995000', '2000-01-01 08:00:00.996000',
               '2000-01-01 08:00:00.997000', '2000-01-01 08:00:00.998000',
               '2000-01-01 08:00:00.999000',        '2000-01-01 08:00:01'],
              dtype='datetime64[ns]', length=1001, freq='L')
>>> pd.date_range(start='2000-01-01 08:00:00', end='2000-01-01 08:00:01', freq=pd.DateOffset(milliseconds=1))
DatetimeIndex(['2000-01-01 08:00:00'], dtype='datetime64[ns]', freq='<DateOffset>')
```

#### Problem description
Today, I can specify 1 millisecond offset in `date_range` by offset alias `1L` or `1ms`. But I cannot specify this offset through `pd.DateOffset` because its lacking a `milliseconds` field.

#### Expected Output
Similar to the case specified with offset alias `L` or `ms`.

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

<details>

INSTALLED VERSIONS
------------------
commit           : 7c48ff4409c622c582c56a5702373f726de08e96
python           : 3.7.10.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-76-generic
Version          : #86-Ubuntu SMP Fri Jan 17 17:24:28 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : en_US.UTF-8

pandas           : 1.2.5
numpy            : 1.21.2
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.2.4
setuptools       : 52.0.0.post20210125
Cython           : 0.29.24
pytest           : 6.2.4
hypothesis       : 6.17.2
sphinx           : 4.1.2
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.3
IPython          : 7.27.0
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : None
fsspec           : 2021.07.0
fastparquet      : None
gcsfs            : None
matplotlib       : 3.4.3
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 5.0.0
pyxlsb           : None
s3fs             : None
scipy            : 1.5.3
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
</details>
