Logout support

[gtsteffaniak]

Add a logout function which calls the logout from provider endpoint based on the well known info.

[soniyadotp]

I was looking for the same, How are managing to implement it right now?

[gtsteffaniak]

I allow users to specify the logout URL in the config and the application makes a request out to it.

Would be great to have go-oidc do it without manual configuration.