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
.module("rank-eval")
        .module("x-pack-ilm")
        .module("x-pack-stack")
        .setting("xpack.security.enabled", "true")
        .setting("xpack.watcher.enabled", "false")
        .setting("xpack.ml.enabled", "false")
        .setting("xpack.license.self_generated.type", "trial")
        .setting("xpack.security.autoconfiguration.enabled", "false")
        .user(USER, PASS)
        .feature(FeatureFlag.TIME_SERIES_MODE)
        .feature(FeatureFlag.DLM_ENABLED)
        .build();
    public CoreWithSecurityClientYamlTestSuiteIT(@Name("yaml") ClientYamlTestCandidate testCandidate) {
        super(testCandidate);
    }
    @ParametersFactory
    public static Iterable<Object[]> parameters() throws Exception {
        return ESClientYamlSuiteTestCase.createParameters();
    }
    @Override
    protected Settings restClientSettings() {
        String token = basicAuthHeaderValue(USER, new SecureString(PASS.toCharArray()));
        return Settings.builder().put(super.restClientSettings()).put(ThreadContext.PREFIX + ".Authorization", token).build();
    }
    @Override
    protected String getTestRestCluster() {
        return cluster.getHttpAddresses();
    }
}