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/plugin/stack/src/main/java/org/elasticsearch/xpack/stack/StackTemplateRegistry.java
package org.elasticsearch.xpack.stack;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.elasticsearch.client.internal.Client;
import org.elasticsearch.cluster.metadata.ComponentTemplate;
import org.elasticsearch.cluster.metadata.ComposableIndexTemplate;
import org.elasticsearch.cluster.service.ClusterService;
import org.elasticsearch.common.settings.Setting;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.threadpool.ThreadPool;
import org.elasticsearch.xcontent.NamedXContentRegistry;
import org.elasticsearch.xcontent.XContentParserConfiguration;
import org.elasticsearch.xcontent.json.JsonXContent;
import org.elasticsearch.xpack.core.ClientHelper;
import org.elasticsearch.xpack.core.ilm.LifecyclePolicy;
import org.elasticsearch.xpack.core.template.IndexTemplateConfig;
import org.elasticsearch.xpack.core.template.IndexTemplateRegistry;
import org.elasticsearch.xpack.core.template.LifecyclePolicyConfig;
import java.io.IOException;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Stream;
public class StackTemplateRegistry extends IndexTemplateRegistry {
    private static final Logger logger = LogManager.getLogger(StackTemplateRegistry.class);
    
    
    public static final int REGISTRY_VERSION = 3;
    public static final String TEMPLATE_VERSION_VARIABLE = "xpack.stack.template.version";