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
@Override
    protected Collection<? extends Plugin> getPlugins() {
        return List.of(new DocumentParserTestsPlugin());
    }
    public void testParseWithRuntimeField() throws Exception {
        DocumentMapper mapper = createDocumentMapper(runtimeFieldMapping(b -> b.field("type", "keyword")));
        ParsedDocument doc = mapper.parse(source(b -> b.field("field", "value")));
        
        assertNull(doc.dynamicMappingsUpdate());
        assertNull(doc.rootDoc().getField("field"));
    }
    public void testParseWithRuntimeFieldArray() throws Exception {
        DocumentMapper mapper = createDocumentMapper(runtimeFieldMapping(b -> b.field("type", "keyword")));
        ParsedDocument doc = mapper.parse(source(b -> b.array("field", "value1", "value2")));
        
        assertNull(doc.dynamicMappingsUpdate());
        assertNull(doc.rootDoc().getField("field"));
    }
    public void testParseWithShadowedField() throws Exception {
        XContentBuilder builder = XContentFactory.jsonBuilder().startObject().startObject("_doc");
        builder.startObject("runtime");
        builder.startObject("field").field("type", "keyword").endObject();
        builder.endObject();