ADD: b2access loginpass

[TomasKulhanek]

B2ACCESS is an easy-to-use and secure Authentication and Authorization platform developed by EUDAT which delivers services for scientific, academia and research infrastructures cross border and cross discipline.

This PR adds oauth2 support for this service.

[lepture]

Thanks. I would prefer to pass the base_url in a function instead of using os.environ. Check https://github.com/authlib/loginpass/blob/master/loginpass/gitlab.py as an example.

[TomasKulhanek]

OK. Agree. I made update for default baseurl. This requires additional client code in case of non-default url, however, conforming the loginpass code style.

[lepture]

And also, can you provide a test case for it?

[TomasKulhanek]

Added test case for b2access and registered in init.py file

[TomasKulhanek]

The fork with this implementation of b2access is used in production for a month now without known problems.