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.endArray();
        }, null, Map.of(field, "points")));
        fields = doc.rootDoc().getFields(field);
        assertThat(fields, hasSize(4));
        assertThat(fields.get(0).fieldType(), sameInstance(LatLonPoint.TYPE));
        assertThat(fields.get(1).fieldType(), sameInstance(LatLonDocValuesField.TYPE));
        assertThat(fields.get(2).fieldType(), sameInstance(LatLonPoint.TYPE));
        assertThat(fields.get(3).fieldType(), sameInstance(LatLonDocValuesField.TYPE));
        doc = mapper.parse(source("1", b -> {
            b.startObject("address");
            b.field("home", "43,-72.34");
            b.endObject();
        }, null, Map.of("address.home", "points")));
        fields = doc.rootDoc().getFields("address.home");
        assertThat(fields, hasSize(2));
        assertThat(fields.get(0).fieldType(), sameInstance(LatLonPoint.TYPE));
        assertThat(fields.get(1).fieldType(), sameInstance(LatLonDocValuesField.TYPE));
    }
    public void testDynamicTemplatesNotFound() throws Exception {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> {
            b.startArray("dynamic_templates");
            {
                b.startObject();
                {
                    b.startObject("booleans");
                    {
                        b.field("match", "none");
                        b.startObject("mapping");
                        {
                            b.field("type", "boolean");