Support for OIDC Logout URL Generation

[johnpudd-ec]

Same issue as shown at workos/workos-go#338, but for the dotnet implementation.

Without the ability to sign out of the id provider, the user never really completes a logout. Although the session maybe ended, the next time the user tries to login, they are automatically logged in with whichever account they used last time (this is experienced with Azure at least).

The user needs to be able to logout fully, requiring a login the next time they return (including the option to use a different account, which is not possible currently).