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/test/java/org/elasticsearch/xpack/stack/StackTemplateRegistryTests.java
import org.elasticsearch.cluster.metadata.ComponentTemplate;
import org.elasticsearch.cluster.metadata.ComposableIndexTemplate;
import org.elasticsearch.cluster.metadata.Metadata;
import org.elasticsearch.cluster.node.DiscoveryNode;
import org.elasticsearch.cluster.node.DiscoveryNodes;
import org.elasticsearch.cluster.node.TestDiscoveryNode;
import org.elasticsearch.cluster.service.ClusterService;
import org.elasticsearch.common.TriFunction;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.test.ClusterServiceUtils;
import org.elasticsearch.test.ESTestCase;
import org.elasticsearch.test.client.NoOpClient;
import org.elasticsearch.test.junit.annotations.TestLogging;
import org.elasticsearch.threadpool.TestThreadPool;
import org.elasticsearch.threadpool.ThreadPool;
import org.elasticsearch.xcontent.NamedXContentRegistry;
import org.elasticsearch.xcontent.ParseField;
import org.elasticsearch.xcontent.XContentParser;
import org.elasticsearch.xcontent.XContentParserConfiguration;
import org.elasticsearch.xcontent.XContentType;
import org.elasticsearch.xpack.core.ilm.DeleteAction;
import org.elasticsearch.xpack.core.ilm.IndexLifecycleMetadata;
import org.elasticsearch.xpack.core.ilm.LifecycleAction;
import org.elasticsearch.xpack.core.ilm.LifecyclePolicy;
import org.elasticsearch.xpack.core.ilm.LifecyclePolicyMetadata;
import org.elasticsearch.xpack.core.ilm.OperationMode;
import org.elasticsearch.xpack.core.ilm.action.PutLifecycleAction;
import org.junit.After;
import org.junit.Before;
import java.io.IOException;