Authn: Handle logout logic in auth broker

[kalleep]

What is this feature?
The logout handler previously had a bunch of provider specific logic when performing logout. This part was a bit messy and hard to follow.

I instead added logout functionality to authn.Service. The service it self perform all shared logic, revoking session token and generate default redirect url.

Some clients (oauth and saml) can be configured with additional logic that should happen. To support this I added a extensions interface that clients can implement authn.LogoutClient. If a client implements this interface it will be called for users that has authenticated with it.

For now it is only implemented for oauth clients. For these we remove access, refresh and id tokens and if configured create a new redirect url with support for oidc rp initiated logout.

Plan is to also fix enterprise saml client to also implement this interface and remove the TODO added here.

Which issue(s) does this PR fix?:

Fixes https://github.com/grafana/identity-access-team/issues/496

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[gamab]

This is truly amazing! Very nice and clean refactor.
I just want to give it a quick test before a final approval.

[gamab]

Tested with AzureAD OIDC Logout, SAML logout, Generic OAuthLogout (with no logout url). Everything worked fine :)