Add OAuth2LoginAuthenticationWebFilter

This is necessary so that the saving of the authorized client occurs
outside of the ReactiveAuthenticationManager. It will allow for
saving with the ServerWebExchange when ReactiveOAuth2AuthorizedClientRepository
is added.

Issue: gh-5621

web/src/main/java/org/springframework/security/web/server/authentication/AuthenticationWebFilter.java

@@ -103,7 +103,7 @@ public class AuthenticationWebFilter implements WebFilter {
 				.onAuthenticationFailure(webFilterExchange, e));
 	}
 
-	private Mono<Void> onAuthenticationSuccess(Authentication authentication, WebFilterExchange webFilterExchange) {
+	protected Mono<Void> onAuthenticationSuccess(Authentication authentication, WebFilterExchange webFilterExchange) {
 		ServerWebExchange exchange = webFilterExchange.getExchange();
 		SecurityContextImpl securityContext = new SecurityContextImpl();
 		securityContext.setAuthentication(authentication);