Logout notification support for HTTP-based authentication mechanisms

I think it makes sense to also allow HTTP mechanisms to handle logouts. Logout is tightly related with authentication and mechanisms should be able to act properly during logout requests.

Although only a few set of mechanisms support logout, I think adding a default method org.wildfly.security.http.HttpServerAuthenticationMechanism#logout will make our API even more complete and capable of supporting more use cases.

The main use case for this enhancement is programmatic logout. In this case, logout can be triggered from inside an application which in turn delegates the logout logic to the mechanism that authenticated an user.

Considering Elytron Web, this enhancement would make integration with other containers even more simple and avoid dealing with specific logout mechanisms (e.g.: notifications) provided by these same containers. This is specially true for servlet containers.