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("country", "de");
                b.field("concrete", "foo");
            }));
            assertNotNull(doc.rootDoc().getField("timestamp"));
            assertNotNull(doc.rootDoc().getField("_source"));
            assertNotNull(doc.rootDoc().getField("location.lat"));
            assertNotNull(doc.rootDoc().getField("location.lon"));
            assertNotNull(doc.rootDoc().getField("concrete"));
            assertNull(doc.rootDoc().getField("country"));
        }
        {
            ParsedDocument doc = mapper.parse(source(b -> {
                b.field("timestamp", "1998-04-30T14:30:17-05:00");
                b.startArray("location");
                {
                    b.startObject().field("lat", 13.5).field("lon", 34.89).endObject();
                    b.startObject().field("lat", 14.5).field("lon", 89.33).endObject();
                }
                b.endArray();
                b.field("country", "de");
                b.field("concrete", "foo");
            }));
            assertNotNull(doc.rootDoc().getField("timestamp"));
            assertNotNull(doc.rootDoc().getField("_source"));
            assertThat(doc.rootDoc().getFields("location.lat"), hasSize(2));
            assertThat(doc.rootDoc().getFields("location.lon"), hasSize(2));
            assertNotNull(doc.rootDoc().getField("concrete"));
            assertNull(doc.rootDoc().getField("country"));
        }
        {