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
}
    public void testWithDynamicTemplates() throws Exception {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> {
            b.startArray("dynamic_templates");
            {
                b.startObject();
                {
                    b.startObject("points");
                    {
                        b.field("match", "none"); 
                        b.startObject("mapping");
                        {
                            b.field("type", "geo_point");
                        }
                        b.endObject();
                    }
                    b.endObject();
                }
                b.endObject();
            }
            b.endArray();
        }));
        String field = randomFrom("loc", "foo.loc", "foo.bar.loc");
        ParsedDocument doc = mapper.parse(source("1", b -> b.field(field, "41.12,-71.34"), null, Map.of(field, "points")));
        List<IndexableField> fields = doc.rootDoc().getFields(field);
        assertThat(fields, hasSize(2));
        assertThat(fields.get(0).fieldType(), sameInstance(LatLonPoint.TYPE));
        assertThat(fields.get(1).fieldType(), sameInstance(LatLonDocValuesField.TYPE));
        doc = mapper.parse(source("1", b -> b.field(field, new double[] { -71.34, 41.12 }), null, Map.of(field, "points")));
        fields = doc.rootDoc().getFields(field);