Enhancing the `logs-*-*` index template with a default ingest pipeline that is first doing a pre-flight check if the `message` field might be JSON and then uses the JSON processor to decode the JSON and merge it top-level with the document.

See also this prototype: https://gist.github.com/felixbarny/a9a2f6243153d5508643fd95ac968a88#file-routing-yml-L114-L174

Open questions and things to consider
* How do users opt-out of default JSON parsing? They could override the index template. Maybe that's good enough.
* Similarly to the logs@custom component template, should we call out to a custom index pipeline? Align with naming in Fleet.
x-pack/plugin/stack/src/test/java/org/elasticsearch/xpack/stack/StackTemplateRegistryTests.java
}
    }
    public void testThatVersionedOldTemplatesAreUpgraded() throws Exception {
        DiscoveryNode node = TestDiscoveryNode.create("node");
        DiscoveryNodes nodes = DiscoveryNodes.builder().localNodeId("node").masterNodeId("node").add(node).build();
        ClusterChangedEvent event = createClusterChangedEvent(
            Collections.singletonMap(
                StackTemplateRegistry.LOGS_SETTINGS_COMPONENT_TEMPLATE_NAME,
                StackTemplateRegistry.REGISTRY_VERSION - 1
            ),
            nodes
        );
        AtomicInteger calledTimes = new AtomicInteger(0);
        client.setVerifier((action, request, listener) -> verifyComponentTemplateInstalled(calledTimes, action, request, listener));
        registry.clusterChanged(event);
        assertBusy(() -> assertThat(calledTimes.get(), equalTo(registry.getComponentTemplateConfigs().size())));
    }
    public void testThatUnversionedOldTemplatesAreUpgraded() throws Exception {
        DiscoveryNode node = TestDiscoveryNode.create("node");
        DiscoveryNodes nodes = DiscoveryNodes.builder().localNodeId("node").masterNodeId("node").add(node).build();
        ClusterChangedEvent event = createClusterChangedEvent(
            Collections.singletonMap(StackTemplateRegistry.LOGS_SETTINGS_COMPONENT_TEMPLATE_NAME, null),
            nodes
        );
        AtomicInteger calledTimes = new AtomicInteger(0);
        client.setVerifier((action, request, listener) -> verifyComponentTemplateInstalled(calledTimes, action, request, listener));
        registry.clusterChanged(event);
        assertBusy(() -> assertThat(calledTimes.get(), equalTo(registry.getComponentTemplateConfigs().size())));
    }
    public void testMissingNonRequiredTemplates() throws Exception {