[python] Add support for paging through callbacks as kwargs

Original issue [here](https://github.com/Azure/autorest.python/issues/665).

Expected syntax for code user:

```python
from azure.core.paging import PagingMethod

class SearchPaging(PagingMethod):
def __init__(self, *args, **kwargs):
# do things

def get_next(self, continuation_token):
# do things
return get_next_return

def extract_data(get_next_return):
# do things
return continuation_token, current_page

pager = client.list_all(paging=SearchPaging(**params))

for item in pager:
print(item)
```

Azure-core impact:
```python
class PagingMethod(abc):

@abc.abstractmethod
def get_next(self, continuation_token):
# do things
return get_next_return

@abc.abstractmethod
def extract_data(get_next_return):
# do things
return continuation_token, current_page

# FIXME
```