Support "FEDERATED" repository types

Currently, calling `get_repositories` on artifactory that contains federated repositories will crash:
```
Traceback (most recent call last):
File "/venv/lib/python3.9/site-packages/artifactory.py", line 2137, in get_repositories
return self._get_all(
File "/venv/lib/python3.9/site-packages/artifactory.py", line 2109, in _get_all
item.read()
AttributeError: 'NoneType' object has no attribute 'read'
```
This happens since `create_by_type` doesn't support the federated type and just returns None.

I can also open a PR on it myself, since the federated type should mostly be handled like a local repository if I understand correctly from the docs:
https://www.jfrog.com/confluence/display/JFROG/Federated+Repositories