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
assertNull(doc.docs().get(1).getField("child.name"));
        assertEquals(10L, doc.docs().get(1).getField("child.age").numericValue());
    }
    public void testRuntimeFieldAndArrayChildren() throws IOException {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> {
            b.field("dynamic", "true");
            b.startObject("runtime");
            {
                b.startObject("object").field("type", "keyword").endObject();
            }
            b.endObject();
        }));
        {
            ParsedDocument doc = mapper.parse(source(b -> {
                b.startObject("object");
                b.array("array", 1, 2, 3);
                b.field("foo", "bar");
                b.endObject();
            }));
            assertNotNull(doc.rootDoc().getField("object.foo"));
            assertNotNull(doc.rootDoc().getField("object.array"));
        }
        {
            ParsedDocument doc = mapper.parse(source(b -> {
                b.startArray("object");
                {
                    b.startObject().array("array", 1, 2, 3).endObject();
                    b.startObject().field("foo", "bar").endObject();
                }
                b.endArray();