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
assertThat(composableIndexTemplate.getIgnoreMissingComponentTemplates(), hasSize(1));
                assertThat(composableIndexTemplate.getIgnoreMissingComponentTemplates().get(0), equalTo("syslog@custom"));
                return AcknowledgedResponse.TRUE;
            } else {
                fail("client called with unexpected request:" + request.toString());
                return null;
            }
        });
        registryWithNonRequiredTemplate.clusterChanged(event);
        assertBusy(() -> assertThat(calledTimes.get(), equalTo(1)));
    }
    @TestLogging(value = "org.elasticsearch.xpack.core.template:DEBUG", reason = "test")
    public void testSameOrHigherVersionTemplateNotUpgraded() {
        DiscoveryNode node = TestDiscoveryNode.create("node");
        DiscoveryNodes nodes = DiscoveryNodes.builder().localNodeId("node").masterNodeId("node").add(node).build();
        Map<String, Integer> versions = new HashMap<>();
        versions.put(StackTemplateRegistry.DATA_STREAMS_MAPPINGS_COMPONENT_TEMPLATE_NAME, StackTemplateRegistry.REGISTRY_VERSION);
        versions.put(StackTemplateRegistry.LOGS_SETTINGS_COMPONENT_TEMPLATE_NAME, StackTemplateRegistry.REGISTRY_VERSION);
        versions.put(StackTemplateRegistry.LOGS_MAPPINGS_COMPONENT_TEMPLATE_NAME, StackTemplateRegistry.REGISTRY_VERSION);
        versions.put(StackTemplateRegistry.METRICS_SETTINGS_COMPONENT_TEMPLATE_NAME, StackTemplateRegistry.REGISTRY_VERSION);
        versions.put(StackTemplateRegistry.METRICS_MAPPINGS_COMPONENT_TEMPLATE_NAME, StackTemplateRegistry.REGISTRY_VERSION);
        versions.put(StackTemplateRegistry.SYNTHETICS_SETTINGS_COMPONENT_TEMPLATE_NAME, StackTemplateRegistry.REGISTRY_VERSION);
        versions.put(StackTemplateRegistry.SYNTHETICS_MAPPINGS_COMPONENT_TEMPLATE_NAME, StackTemplateRegistry.REGISTRY_VERSION);
        ClusterChangedEvent sameVersionEvent = createClusterChangedEvent(versions, nodes);
        client.setVerifier((action, request, listener) -> {
            if (action instanceof PutComponentTemplateAction) {
                fail("template should not have been re-installed");
                return null;
            } else if (action instanceof PutLifecycleAction) {