Currently, when an object is mapped with `subobjects: false`, trying to ingest a document that has nested objects leads to a mapping exception.

In a couple of occasions, we've been talking about that it would be convenient if Elasticsearch could automatically flatten the fields instead of the client having to know about the `subobjects` setting. This would make `subobjects: false` more of a drop-in replacement that allows for a much easier adoption, without changes in the client.

Note that the `_source` should ideally stay untouched, just the stored fields should be flattened automatically.

Some related issues
* https://github.com/elastic/apm/issues/347
* https://github.com/elastic/elasticsearch/issues/88777
* https://github.com/elastic/package-spec/issues/349
server/src/test/java/org/elasticsearch/index/mapper/DocumentParserTests.java
ParsedDocument doc = docMapper.parse(source(b -> b.field("runtime.foo", 10)));
        Mapping mapping = doc.dynamicMappingsUpdate();
        assertNotNull(mapping);
        Mapper xMapper = mapping.getRoot().getMapper("runtime");
        assertNull(xMapper);
        assertNotNull(mapping.getRoot().getRuntimeField("runtime.foo"));
    }
    public void testMultipleSubfieldMappingUpdate() throws Exception {
        DocumentMapper docMapper = createDummyMapping();
        ParsedDocument doc = docMapper.parse(source(b -> {
            b.field("x.foo", 10);
            b.field("x.bar", 20);
        }));
        Mapping mapping = doc.dynamicMappingsUpdate();
        assertNotNull(mapping);
        Mapper xMapper = mapping.getRoot().getMapper("x");
        assertNotNull(xMapper);
        assertTrue(xMapper instanceof ObjectMapper);
        assertNotNull(((ObjectMapper) xMapper).getMapper("foo"));
        assertNotNull(((ObjectMapper) xMapper).getMapper("bar"));
        assertNull(((ObjectMapper) xMapper).getMapper("subx"));
    }
    public void testDeepSubfieldMappingUpdate() throws Exception {
        DocumentMapper docMapper = createDummyMapping();
        ParsedDocument doc = docMapper.parse(source(b -> b.field("x.subx.foo", 10)));
        Mapping mapping = doc.dynamicMappingsUpdate();
        assertNotNull(mapping);
        Mapper xMapper = mapping.getRoot().getMapper("x");
        assertNotNull(xMapper);
        assertTrue(xMapper instanceof ObjectMapper);