feat(session): support headers and don't set username/password if none

[ahmet2mir]

Hello,

In order to support all differents auth provided by Confluence, i simply ignore username/password inside session and allow adding extra headers with config to allow all supported auth.

When we set headers on requests session, they are "merged" with headers on session requests.

Example

$ python -m http.server
$ python
Python 3.9.5 (default, May 27 2021, 16:11:10) 
>>> import requests
>>> session = requests.Session()
>>> session.headers={"x-test-var": "hello"}
>>> r = session.get("http://localhost:8000", headers={"x-other-header": "world"})
>>> r.request.headers
{'x-test-var': 'hello', 'x-other-header': 'world'}