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/core/src/test/java/org/elasticsearch/xpack/core/template/IndexTemplateRegistryTests.java
);
        registry.clusterChanged(event);
        assertBusy(() -> assertThat(calledTimes.get(), equalTo(4)));
    }
    public void testThatTemplatesAreNotUpgradedWhenNotNeeded() throws Exception {
        DiscoveryNode node = TestDiscoveryNode.create("node");
        DiscoveryNodes nodes = DiscoveryNodes.builder().localNodeId("node").masterNodeId("node").add(node).build();
        AtomicInteger calledTimes = new AtomicInteger(0);
        client.setVerifier((action, request, listener) -> {
            if (action instanceof PutComposableIndexTemplateAction) {
                
                return AcknowledgedResponse.TRUE;
            } else {
                fail("client called with unexpected request: " + request.toString());
                return null;
            }
        });
        ClusterChangedEvent event = createClusterChangedEvent(
            Map.of("custom-plugin-settings", 3),
            Collections.emptyMap(),
            Map.of("custom-plugin-default_pipeline", 3, "custom-plugin-final_pipeline", 3),
            nodes
        );
        registry.clusterChanged(event);
        assertBusy(() -> assertThat(calledTimes.get(), equalTo(0)));
    }
    private static void assertPutComponentTemplate(
        AtomicInteger calledTimes,
        ActionType<?> action,
        ActionRequest request,