Rethink authentication configuration

Right now, the server supports two different auth modes, confusingly named auth: "tls" and auth: "oidc", even though that doesn't cover exactly what they do. The main problem is that this one setting gives the impression that both methods are mutually exclusive, whereas they are not.

Discussions such as #667, #671, https://github.com/solid/solid-auth-client/issues/44, https://github.com/solid/solid/issues/138, https://github.com/solid/oidc-auth-manager/issues/20 show that there is a need for more clarity, and a better configuration option.

This issue tracks general progress and will act as a tracker for multiple sub-issues.
Here are the steps we need to take in my opinion:

• Have one single contract for the entire node-solid-server to track a) the WebID of the logged-in agent b) the method through which that agents has logged in
• Document the above contract
• Ensure that “ask for a client certificate on every resource (WebID-TLS)” can be enabled and disabled independently of other features
• Ensure that “act as an OIDC identity provider” can be enabled and disabled independently of other features
• Ensure that “log in with OIDC through username/password” can be enabled and disabled independently of other features
• Ensure that “log in with OIDC through client certificates (WebID-TLS)” can be enabled and disabled independently of other features
• Ensure that “accept bearer tokens” can be enabled and disabled independently of other features
• Examine whether we want to always translate successful logins into an Express cookie-based session
• Examine whether we want to allow turning on or off cookie-based sessions
• Create a configuration section that allows to enable and disable all of the above options independently
• Update the init generator to create the right config
• Remove the old auth config section

Feedback welcome.

Start by reading discussions #667, #671, and the linked authentication issues to understand the unresolved design. Then trace the existing auth configuration and init generator, both named in the issue. Done means replacing the old auth section with documented independent controls for the listed login, token, certificate, identity-provider, and cookie-session options.