Allow multiple ServerLogoutHandler instances in WebFlux

[blake-bauman]

Spring Security for Spring MVC allows for specifying multiple LogoutHandler implementations which get wrapped in a DelegatingLogoutHandler. Spring Security for WebFlux currently only allows a single ServerLogoutHandler implementation.

This PR puts both Spring MVC and Spring WebFlux on equal functionality when it comes to logout handlers.

[jzheaux]

Thanks, @blake-bauman, for the PR! In addition to my inline feedback, will you please add some tests to confirm that the new functionality works?

[jzheaux]

Instead of exposing addLogoutHandler, would logout(Consumer<List<ServerLogoutHandler>> consumer) also service your needs? The reason this is nice it because it also allows you to remove values. Please see OneTimeTokenLogoutSpec#authenticationSuccessHandler for an example.

[jzheaux]

Will you please add @since 7.0

[jzheaux]

@blake-bauman, please also make sure to sign your commit and keep the commit title to about 50 characters, for example:

Support Multiple ServerLogoutHandlers

This commit adds support to ServerHttpSecurity for registering
multiple ServerLogoutHandlers. This is handy so that an application
does not need to re-supply any handlers already configured by
the DSL

[blake-bauman]

I'm a little confused because I did sign it using the -s

[blake-bauman]

@jzheaux I didn't see any existing tests for the Logout handler. Could you point to where they might be so that I can add on?

[blake-bauman]

Instead of exposing addLogoutHandler, would logout(Consumer<List> consumer) also service your needs?

@jzheaux The reason I did it this way is because LogoutConfigurer for Servlet has addLogoutHandler() as public, and was attempting to match the spec for consistency. However, if you'd prefer that I switch to using Consumer<List<...>>, I can do that instead.