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
{"foo":{"type":"keyword"}}""", Strings.toString(foo));
    }
    public void testDynamicRuntimeBooleanArray() throws Exception {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> b.field("dynamic", "runtime")));
        ParsedDocument doc = mapper.parse(source(b -> b.startArray("foo").value(true).value(false).endArray()));
        assertEquals(0, doc.rootDoc().getFields("foo").size());
        RuntimeField foo = doc.dynamicMappingsUpdate().getRoot().getRuntimeField("foo");
        assertEquals("""
            {"foo":{"type":"boolean"}}""", Strings.toString(foo));
    }
    public void testDynamicRuntimeDateArray() throws Exception {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> b.field("dynamic", "runtime")));
        ParsedDocument doc = mapper.parse(source(b -> b.startArray("foo").value("2020-12-15").value("2020-12-09").endArray()));
        assertEquals(0, doc.rootDoc().getFields("foo").size());
        RuntimeField foo = doc.dynamicMappingsUpdate().getRoot().getRuntimeField("foo");
        assertEquals("""
            {"foo":{"type":"date"}}""", Strings.toString(foo));
    }
    public void testMappedGeoPointArray() throws Exception {
        DocumentMapper mapper = createDocumentMapper(fieldMapping(b -> {
            b.field("type", "geo_point");
            b.field("doc_values", false);
        }));
        ParsedDocument doc = mapper.parse(source(b -> {
            b.startArray("field");
            {
                b.startArray().value(0).value(0).endArray();
                b.startArray().value(1).value(1).endArray();
            }
            b.endArray();