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
DATA_STREAMS_MAPPINGS_COMPONENT_TEMPLATE_NAME,
                "/data-streams-mappings.json",
                REGISTRY_VERSION,
                TEMPLATE_VERSION_VARIABLE
            ),
            new IndexTemplateConfig(
                LOGS_MAPPINGS_COMPONENT_TEMPLATE_NAME,
                "/logs-mappings.json",
                REGISTRY_VERSION,
                TEMPLATE_VERSION_VARIABLE
            ),
            new IndexTemplateConfig(
                LOGS_SETTINGS_COMPONENT_TEMPLATE_NAME,
                "/logs-settings.json",
                REGISTRY_VERSION,
                TEMPLATE_VERSION_VARIABLE
            ),
            new IndexTemplateConfig(
                METRICS_MAPPINGS_COMPONENT_TEMPLATE_NAME,
                "/metrics-mappings.json",
                REGISTRY_VERSION,
                TEMPLATE_VERSION_VARIABLE
            ),
            new IndexTemplateConfig(
                METRICS_SETTINGS_COMPONENT_TEMPLATE_NAME,
                "/metrics-settings.json",
                REGISTRY_VERSION,
                TEMPLATE_VERSION_VARIABLE
            ),
            new IndexTemplateConfig(