Allow to set an authentication manager for back-channel logout in ServerHttpSecurity

[ErwinSteffens]

Allow to set custom OidcBackChannelLogoutReactiveAuthenticationManager instance so you can override the logout token decoder factory with for setting a custom WebClient instance.

Issue: #16545

Usage:

http
		.oauth2Login(Customizer.withDefaults())
		.oidcLogout(customizer -> customizer
				.backChannel(config -> {
					var manager = new OidcBackChannelLogoutReactiveAuthenticationManager();
					manager.setLogoutTokenDecoderFactory(logoutTokenDecoderFactory);
					config.authenticationManager(manager);
				}));