Add AuthenticationProvider List Configurability

Issue gh-5185

Author: jzheaux

config/src/main/java/org/springframework/security/config/http/AuthenticationConfigBuilder.java

@@ -167,6 +167,7 @@ final class AuthenticationConfigBuilder {
 	private BeanDefinition authorizationCodeGrantFilter;
 	private BeanReference authorizationCodeAuthenticationProviderRef;
 
+	private final List<BeanReference> authenticationProviders = new ManagedList<>();
 	private final Map<BeanDefinition, BeanMetadataElement> defaultDeniedHandlerMappings = new ManagedMap<>();
 	private final Map<BeanDefinition, BeanMetadataElement> defaultEntryPointMappings = new ManagedMap<>();
 	private final List<BeanDefinition> csrfIgnoreRequestMatchers = new ManagedList<>();
@@ -1013,6 +1014,8 @@ List<BeanReference> getProviders() {
 			providers.add(authorizationCodeAuthenticationProviderRef);
 		}
 
+		providers.addAll(this.authenticationProviders);
+
 		return providers;
 	}