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
b.field("dynamic", "true");
            b.startObject("runtime");
            {
                b.startObject("child.name").field("type", "keyword").endObject();
            }
            b.endObject();
            b.startObject("properties");
            {
                b.startObject("child");
                {
                    b.field("type", "nested");
                }
                b.endObject();
            }
            b.endObject();
        }));
        ParsedDocument doc = mapper.parse(source(b -> {
            b.field("name", "alice");
            b.field("age", 42);
            b.startArray("child");
            {
                b.startObject().field("name", "bob").field("age", 12).endObject();
                b.startObject().field("name", "charlie").field("age", 10).endObject();
            }
            b.endArray();
        }));
        assertEquals(3, doc.docs().size());
        assertEquals("alice", doc.rootDoc().getField("name").stringValue());
        assertNull(doc.docs().get(0).getField("child.name"));   
        assertEquals(12L, doc.docs().get(0).getField("child.age").numericValue());