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
}));
            assertNotNull(doc.rootDoc().getField("object.foo"));
            assertNotNull(doc.rootDoc().getField("object.array"));
        }
    }
    public void testRuntimeFieldDoesNotShadowObjectChildren() throws IOException {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> {
            b.field("dynamic", "true");
            b.startObject("runtime");
            {
                b.startObject("location").field("type", "keyword").endObject();
                b.startObject("country").field("type", "keyword").endObject();
            }
            b.endObject();
            b.startObject("properties");
            {
                b.startObject("timestamp").field("type", "date").endObject();
                b.startObject("concrete").field("type", "keyword").endObject();
            }
            b.endObject();
        }));
        {
            ParsedDocument doc = mapper.parse(source(b -> {
                b.field("timestamp", "1998-04-30T14:30:17-05:00");
                b.startObject("location");
                {
                    b.field("lat", 13.5);
                    b.field("lon", 34.89);
                }
                b.endObject();