Call out to a missing `@custom` pipeline so that users can add custom processors to the default logs pipeline.

Open questions
* Can we make the custom pipeline specific for a dataset?
* Should this pipeline also be called by integrations?
* Align with https://github.com/elastic/kibana/issues/149484

Relates to https://github.com/elastic/elasticsearch/pull/95481

cc @joshdover @kpollich 
x-pack/qa/core-rest-tests-with-security/src/yamlRestTest/java/org/elasticsearch/xpack/security/CoreWithSecurityClientYamlTestSuiteIT.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */
package org.elasticsearch.xpack.security;
import com.carrotsearch.randomizedtesting.annotations.Name;
import com.carrotsearch.randomizedtesting.annotations.ParametersFactory;
import com.carrotsearch.randomizedtesting.annotations.TimeoutSuite;
import org.apache.lucene.tests.util.TimeUnits;
import org.elasticsearch.common.settings.SecureString;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.util.concurrent.ThreadContext;
import org.elasticsearch.test.cluster.ElasticsearchCluster;
import org.elasticsearch.test.cluster.FeatureFlag;
import org.elasticsearch.test.rest.yaml.ClientYamlTestCandidate;
import org.elasticsearch.test.rest.yaml.ESClientYamlSuiteTestCase;
import org.junit.ClassRule;
import java.util.Objects;
@TimeoutSuite(millis = 30 * TimeUnits.MINUTE) 
public class CoreWithSecurityClientYamlTestSuiteIT extends ESClientYamlSuiteTestCase {
    private static final String USER = Objects.requireNonNull(System.getProperty("tests.rest.cluster.username", "test_admin"));
    private static final String PASS = Objects.requireNonNull(System.getProperty("tests.rest.cluster.password", "x-pack-test-password"));
    @ClassRule
    public static ElasticsearchCluster cluster = ElasticsearchCluster.local()
        .module("constant-keyword")
        .module("mapper-extras")